Merge #5597

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

- Related: #4971

**2. Or, if no issue exists, describe the change:**

_If applicable, please follow the issue templates to provide as much detail as
possible._

**Problem:**
A2A support lacked pluggable or persistent task store backends, forcing a strict default to `InMemoryTaskStore`.

**Solution:**
Extended `ServiceRegistry` and `ServiceFactory` to support URI-driven configuration for A2A task stores. Added built-in support for `memory://`, `postgresql://`, `mysql://`, and `sqlite://` schemes. Plumbed the options down into `to_a2a()` and `get_fast_api_app()`, ensuring connection strings are securely redacted from application logs.

### Testing Plan

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

Summary: Ran `pytest` against `test_agent_to_a2a.py`, `test_fast_api.py`, `test_service_registry.py`, and `test_service_factory.py`. All 131 unit tests passed successfully.

**Manual End-to-End (E2E) Tests:**

Compiled the package distribution wheel via `uv build` and successfully validated its installation inside a clean isolated sandbox environment via `uv pip install dist/google_adk-1.32.0-py3-none-any.whl[a2a]`. Verified that the application instantiates default in-memory stores and custom persistent URI stores

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.

COPYBARA_INTEGRATE_REVIEW=#5597 from allen-stephen:feat/a2a-task-store-support f53ba0f
PiperOrigin-RevId: 914496800