DatabaseSessionService throws internal server error when creating the same session twice

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[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Describe the bug**
Creating a session twice with the same id for the same user returns an internal server error when using a `DatabaseSessionService`.

**To Reproduce**
Start an ADK server. Here's a minimal example:
```python
import os
import uvicorn
from dotenv import find_dotenv, load_dotenv
from fastapi import FastAPI
from google.adk.cli.fast_api import get_fast_api_app

load_dotenv(find_dotenv())

# --- Configuration ---
AGENT_DIR = os.path.dirname(os.path.abspath(__file__))
SESSION_SERVICE_URI = (
    f"postgresql://{os.getenv('DB_USERNAME')}:{os.getenv('DB_PASSWORD')}@"
    f"{os.getenv('DB_HOST')}:{os.getenv('DB_PORT')}/{os.getenv('DB_NAME')}"
)

# --- Set up the FastAPI application ---
app: FastAPI = get_fast_api_app(
    agents_dir=AGENT_DIR,
    session_service_uri=SESSION_SERVICE_URI,
    web=True,
)


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8081)
```

And a very simple agent:
```python
import os
from google.adk import Agent

AGENT_NAME = "hello_world_agent"
 
root_agent = Agent(
    name=AGENT_NAME,
    model=os.getenv("MODEL_AGENT"),
    instruction="Just say hello back to the user.",
)
```

Supposing the database is correctly set up, creating a session can be done like this:
```
curl -X POST "http://localhost:8081/apps/hello_world_agent/users/user/sessions" -H "Content-Type: application/json" -d '{"session_id": "test-123"}'
```

When sending the same request again, I would expect the server to have the same behavior as when using the `InMemorySessionService`, but it is not the case. Instead, an error is thrown:

```
INFO:     127.0.0.1:61218 - "POST /apps/hello_world_agent/users/user/sessions HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1967, in _exec_single_context
    self.dialect.do_execute(
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/default.py", line 951, in do_execute
    cursor.execute(statement, parameters)
psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "sessions_pkey"
DETAIL:  Key (app_name, user_id, id)=(hello_world_agent, user, test-123) already exists.


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 63, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/routing.py", line 716, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/routing.py", line 736, in app
    await route.handle(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/routing.py", line 290, in handle
    await self.app(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/routing.py", line 78, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/starlette/routing.py", line 75, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/fastapi/routing.py", line 302, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/fastapi/routing.py", line 213, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/google/adk/cli/adk_web_server.py", line 524, in create_session
    session = await self.session_service.create_session(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/google/adk/sessions/database_session_service.py", line 488, in create_session
    sql_session.commit()
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 2032, in commit
    trans.commit(_to_root=True)
  File "<string>", line 2, in commit
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/state_changes.py", line 137, in _go
    ret_value = fn(self, *arg, **kw)
                ^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 1313, in commit
    self._prepare_impl()
  File "<string>", line 2, in _prepare_impl
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/state_changes.py", line 137, in _go
    ret_value = fn(self, *arg, **kw)
                ^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 1288, in _prepare_impl
    self.session.flush()
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 4345, in flush
    self._flush(objects)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 4480, in _flush
    with util.safe_reraise():
         ^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/util/langhelpers.py", line 224, in __exit__
    raise exc_value.with_traceback(exc_tb)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 4441, in _flush
    flush_context.execute()
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/unitofwork.py", line 466, in execute
    rec.execute(self)
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/unitofwork.py", line 642, in execute
    util.preloaded.orm_persistence.save_obj(
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/persistence.py", line 93, in save_obj
    _emit_insert_statements(
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/orm/persistence.py", line 1233, in _emit_insert_statements
    result = connection.execute(
             ^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1419, in execute
    return meth(
           ^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/sql/elements.py", line 526, in _execute_on_connection
    return connection._execute_clauseelement(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1641, in _execute_clauseelement
    ret = self._execute_context(
          ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1846, in _execute_context
    return self._exec_single_context(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1986, in _exec_single_context
    self._handle_dbapi_exception(
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 2355, in _handle_dbapi_exception
    raise sqlalchemy_exception.with_traceback(exc_info[2]) from e
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1967, in _exec_single_context
    self.dialect.do_execute(
  File "/Users/user/LocalProjects/agent_project/adk/.venv/lib/python3.12/site-packages/sqlalchemy/engine/default.py", line 951, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (psycopg2.errors.UniqueViolation) duplicate key value violates unique constraint "sessions_pkey"
DETAIL:  Key (app_name, user_id, id)=(hello_world_agent, user, test-123) already exists.

[SQL: INSERT INTO sessions (app_name, user_id, id, state, create_time, update_time) VALUES (%(app_name)s, %(user_id)s, %(id)s, %(state)s::JSONB, now(), now()) RETURNING sessions.create_time, sessions.update_time]
[parameters: {'app_name': 'hello_world_agent', 'user_id': 'user', 'id': 'test-123', 'state': '{}'}]
(Background on this error at: https://sqlalche.me/e/20/gkpj)
```

**Expected behavior**
Just like when using the `InMemorySessionService`, no error should be thrown.

**Desktop (please complete the following information):**
 - OS: macOS
 - Python version(python -V): 3.12.9
 - ADK version(pip show google-adk): 1.14.0

 **Model Information:**
 - Are you using LiteLLM: No
 - Which model is being used: gemini-2.5


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DatabaseSessionService throws internal server error when creating the same session twice #2950

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DatabaseSessionService throws internal server error when creating the same session twice #2950

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