Update ADK doc according to issue #1292 - 9 - DatabaseSessionService Concurrency and Locking

[adk-bot]

9. Document concurrency and locking improvements in DatabaseSessionService.

Doc file: docs/sessions/session/index.md

Current state:

The documentation for DatabaseSessionService mentions it connects to relational databases and requires an async driver. It doesn't mention concurrency handling.

Proposed Change:

Add a note about Concurrency and Locking under the DatabaseSessionService section.
Explain that the service now implements internal locking (in-process) and uses row-level locking (for PostgreSQL, MySQL, MariaDB) to prevent race conditions when multiple requests try to update the same session simultaneously. This ensures data consistency in concurrent environments.

Reasoning:
Significant improvements were made to DatabaseSessionService to handle concurrency and locking, which is critical for production deployments. Users should be aware of these robustness features.

Reference: src/google/adk/sessions/database_session_service.py

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