feat: add FirestoreSessionService for serverless session persistence

Adds a new session service backed by Google Cloud Firestore, providing
persistent, serverless session storage. This addresses the gap between
InMemorySessionService (dev-only) and DatabaseSessionService (requires
managing SQL infrastructure) for GCP-native deployments.

The implementation follows established ADK patterns:
- Extends BaseSessionService with all CRUD operations
- Supports three-tier state management (app/user/session)
- Events stored in Firestore subcollections (avoids 1MB doc limit)
- Lazy import via __getattr__ (same pattern as DatabaseSessionService)
- google-cloud-firestore added as optional 'firestore' extra

Ref: #3776
Co-authored-by: Cursor <[email protected]>

Author: anmolg1997

pyproject.toml

@@ -164,6 +164,8 @@ extensions = [
   "toolbox-adk>=0.5.7, <0.6.0",                 # For tools.toolbox_toolset.ToolboxToolset
 ]
 
+firestore = ["google-cloud-firestore>=2.19.0, <3.0.0"]
+
 otel-gcp = ["opentelemetry-instrumentation-google-genai>=0.6b0, <1.0.0"]
 
 toolbox = ["toolbox-adk>=0.5.7, <0.6.0"]

src/google/adk/sessions/__init__.py

@@ -20,6 +20,7 @@
 __all__ = [
     'BaseSessionService',
     'DatabaseSessionService',
+    'FirestoreSessionService',
     'InMemorySessionService',
     'Session',
     'State',
@@ -38,4 +39,14 @@ def __getattr__(name: str):
           'DatabaseSessionService requires sqlalchemy>=2.0, please ensure it is'
           ' installed correctly.'
       ) from e
+  if name == 'FirestoreSessionService':
+    try:
+      from .firestore_session_service import FirestoreSessionService
+
+      return FirestoreSessionService
+    except ImportError as e:
+      raise ImportError(
+          'FirestoreSessionService requires google-cloud-firestore, please'
+          ' install it with: pip install google-cloud-firestore'
+      ) from e
   raise AttributeError(f'module {__name__!r} has no attribute {name!r}')