fix(sessions): use INSERT ON CONFLICT DO NOTHING to prevent race in create_session

[Lyt060814]

Description

DatabaseSessionService.create_session contains a SELECT-then-INSERT pattern when initializing app_states and user_states rows for a new app_name/user_id. Under concurrent parallel calls (e.g. multiple sub-agents starting simultaneously), all callers see None from the SELECT and all attempt INSERT, causing a UniqueViolation on the primary key constraint.

Fix

Replace the check-then-insert pattern with dialect-specific INSERT ... ON CONFLICT DO NOTHING:

• PostgreSQL: Uses sqlalchemy.dialects.postgresql.insert with on_conflict_do_nothing(index_elements=[...])
• SQLite: Uses sqlalchemy.dialects.sqlite.insert with on_conflict_do_nothing()
• Other dialects: Falls back to the original get-then-add pattern for backward compatibility

The upsert is followed by a regular SELECT to fetch the (now guaranteed to exist) state rows for subsequent state delta application.

Tests

Added 2 regression tests:

• test_concurrent_create_session_no_unique_violation: 10 concurrent create_session calls with same app_name, different user_ids (tests the app_states race)
• test_concurrent_create_session_same_user_no_unique_violation: 10 concurrent calls with same app_name AND user_id (tests the user_states race)

All 80 existing session tests pass.

Files Changed

• src/google/adk/sessions/database_session_service.py: Added _ensure_state_rows() method, updated create_session() to use it
• tests/unittests/sessions/test_session_service.py: Added 2 regression tests

Fixes #4954

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[rohityan]

Hi @Lyt060814 , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!

[Lyt060814]

Hi @rohityan, thanks for the heads up! CLA is now signed and the check is passing ✅. I've also rebased onto the latest main to bring the branch up to date. Ready for review whenever you get a chance!