feat: add BufferableSessionService

PiperOrigin-RevId: 912057968

Author: google-genai-bot

src/google/adk/runners.py

@@ -1616,6 +1616,10 @@ async def close(self):
     if self.plugin_manager:
       await self.plugin_manager.close()
 
+    # Close Session Service
+    if self.session_service:
+      await self.session_service.flush()
+
     logger.info('Runner closed.')
 
   if sys.version_info < (3, 11):

src/google/adk/sessions/base_session_service.py

@@ -124,6 +124,13 @@ async def append_event(self, session: Session, event: Event) -> Event:
     session.events.append(event)
     return event
 
+  async def flush(self):
+    """Flushes any buffered events.
+
+    For non-buffering implementations, this can be a no-op.
+    """
+    pass
+
   def _apply_temp_state(self, session: Session, event: Event) -> None:
     """Applies temp-scoped state delta to the in-memory session state.