feat(sessions): add BufferedFirestoreSessionService

Add a Firestore-backed session service that buffers events in memory and
flushes them in a single transaction per session, collapsing the repeated
session-doc/state-doc updates and per-event transactions from N to 1.

Mirrors the data model of the builtin
google.adk.integrations.firestore.FirestoreSessionService (collection
hierarchy, app/user/session state scoping, optimistic concurrency via a
revision field, idempotent event docs keyed by event.id) and adds:
- in-memory per-session buffering with count/interval/explicit/shutdown flush
- durable_mode to persist every event immediately
- exponential backoff with jitter on retryable errors
- start()/stop()/flush() ADK lifecycle hooks

Gated behind the new optional [firestore] extra. Unit tests use an in-memory
fake AsyncClient (no external services).

Author: enesdemirag

pyproject.toml

@@ -39,6 +39,9 @@ changelog = "https://github.com/google/adk-python-community/blob/main/CHANGELOG.
 documentation = "https://google.github.io/adk-docs/"
 
 [project.optional-dependencies]
+firestore = [
+    "google-cloud-firestore>=2.11.0, <3.0.0",  # For BufferedFirestoreSessionService
+]
 s3 = [
     "aioboto3>=13.0.0",  # For S3ArtifactService
 ]

src/google/adk_community/sessions/__init__.py

@@ -14,6 +14,7 @@
 
 """Community session services for ADK."""
 
+from .firestore_session_service import BufferedFirestoreSessionService
 from .redis_session_service import RedisSessionService
 
-__all__ = ["RedisSessionService"]
+__all__ = ["BufferedFirestoreSessionService", "RedisSessionService"]