feat: add FirestoreSessionService for serverless session persistence

[anmolg1997]

Summary

Adds a FirestoreSessionService — a BaseSessionService implementation backed by Google Cloud Firestore, providing persistent, serverless session storage for Cloud Run, Cloud Functions, and other GCP environments where managing a SQL database is undesirable.

This was originally submitted as google/adk-python#4439, where maintainer @rohityan directed it to this community repo.

Changes

• src/google/adk_community/sessions/firestore_session_service.py — full implementation
• tests/unittests/sessions/test_firestore_session_service.py — 19 unit tests with lightweight in-memory Firestore mock (no GCP project required)
• src/google/adk_community/sessions/__init__.py — export FirestoreSessionService
• pyproject.toml — add firestore optional dependency group

Design Decisions

Concern	Approach
1MB doc limit	Events stored in Firestore subcollections, not embedded
Race conditions	App/user state updates use Firestore transactions
N+1 in list_sessions	Shared app/user state fetched once outside the loop
Bulk event deletion	AsyncWriteBatch in 500-doc chunks
num_recent_events	Server-side limit_to_last() instead of client-side slicing
Redundant reads	create_session reuses state returned from transactional updates
Optional dependency	google-cloud-firestore imported at runtime with clear error message
State management	Three-tier (app/user/session) matching ADK conventions, temp state filtered

Addresses Review Feedback

All issues raised by Gemini Code Assist on the original PR have been addressed:

• ✅ High: Race conditions in create_session and append_event → Firestore transactions
• ✅ High: N+1 query in list_sessions → single shared fetch
• ✅ Medium: Redundant state re-fetches → reuse transaction return values
• ✅ Medium: Client-side slicing for recent events → limit_to_last()
• ✅ Medium: One-by-one event deletion → AsyncWriteBatch with chunking

Test Plan

• 19 unit tests covering all CRUD operations, state management, filtering, error cases
• All tests pass without a GCP project (in-memory mock)
• Existing Redis tests unaffected (1 pre-existing failure in test_session_state_management on main)
• Manual validation with a real Firestore instance

Fixes #103
Ref: google/adk-python#3776