refactor(firestore) : comprehensive refactoring for FirestoreSessionService

[Dumeng]

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Problem:
This Pull Request introduces comprehensive refactoring, performance optimizations, and robustness improvements to the FirestoreSessionService. It simplifies the core implementation, aligns state handling logic with other session services, fixes side-effects in transactions, and significantly improves read performance for bulk operations.

Solution:

• Optimized Batch Retrieval: Updated list_sessions to leverage Firestore's native client.get_all(...) method. This fetches multiple user state documents in a single optimized RPC request rather than individual sequential reads, significantly reducing read latency and overhead.

• Aligned State Handling: Refactored state management logic to match the established, robust patterns found in DatabaseSessionService.
  Transaction Safety: Removed unsafe side-effects inside transaction callbacks within append_event to ensure idempotency and reliability during automatic Firestore transaction retries.

• Revision Tracking: Explicitly initialized session revision tracking to 0 upon session creation to guarantee monotonic tracking consistency.

• Simplified Implementation: Streamlined internal data extraction and simplified redundant record lookups for improved long-term maintainability.

• Import Standards: Resorted and updated import statements to strictly adhere to the ADK project's relative import conventions and code style guide.

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• 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.

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