feat(a2a): add support for persistent task stores

[allen-stephen]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

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

• Related: get_fast_api_app does not allow injecting a custom A2A task store #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:

• I have added or updated unit tests for my change.
• 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

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.