feat: Support for Database based Push Config Store

[kthota-g]

Description

Introduces support for Database backed Push Notification Config Store.

Push Notification Config persisted to the database table gets encrypted when encryption_key param is set for the DatabasePushNotificationConfigStore.

Usage:

httpx_client = httpx.AsyncClient()
 engine = create_async_engine(
            "sqlite+aiosqlite:///file::memory:?cache=shared",
            echo=False
        )

push_config_store = DatabasePushNotificationConfigStore(engine=engine,
                    encryption_key=Fernet.generate_key()) 
push_sender = BasePushNotificationSender(
    httpx_client=httpx_client,
    config_store=push_config_store)

 request_handler = DefaultRequestHandler(
            agent_executor=CurrencyAgentExecutor(),
            task_store=DatabaseTaskStore(engine=engine),
            push_config_store=push_config_store,
            push_sender=push_sender)

[gemini-code-assist]

Summary of Changes

Hello @kthota-g, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the persistence and security of push notification configurations by introducing a robust database-backed storage solution. It allows for the secure storage of configuration data in various SQL databases, with an added layer of optional encryption for sensitive information.

Highlights

• Database-backed Push Notification Config Store: Introduced a new DatabasePushNotificationConfigStore that leverages SQLAlchemy to persist push notification configurations in a database. This replaces or complements in-memory storage, providing durability and scalability.
• Optional Data Encryption: The DatabasePushNotificationConfigStore now supports optional encryption of sensitive config_data using cryptography.fernet. If an encryption_key is provided during initialization, data is encrypted before being written to the database and decrypted upon retrieval.
• Configurable Database Model: A new PushNotificationConfigMixin and create_push_notification_config_model function were added to src/a2a/server/models.py, allowing the database table for push notification configurations to be named dynamically, enhancing flexibility for different deployment scenarios.
• Comprehensive Test Coverage: Extensive new tests have been added for the DatabasePushNotificationConfigStore, covering CRUD operations, initialization idempotency, and critical encryption/decryption scenarios across various database backends (SQLite, PostgreSQL, MySQL).

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[gemini-code-assist]

Code Review

This pull request introduces a DatabasePushNotificationConfigStore for persisting push notification configurations, with optional encryption using the cryptography library. The implementation is well-structured, with good error handling and comprehensive tests.

I've identified a correctness issue in the decryption logic that could lead to an unhandled exception, and a couple of issues in the tests where they don't correctly test the intended functionality. My review includes suggestions to fix these issues.

Overall, this is a solid feature addition.