fix: exclude temp: state keys from Firestore session writes

FirestoreSessionService.append_event() writes the full session.state
dict to Firestore, filtering app: and user: prefixed keys but not
temp: prefixed keys. This causes crashes when temp state contains
non-serializable objects (e.g. GroundingMetadata from GoogleSearchTool
via AgentTool's propagate_grounding_metadata).

The DatabaseSessionService and SqliteSessionService don't have this
issue because they only persist state_deltas extracted via
extract_state_delta(), which already filters temp: keys.

Fix: add State.TEMP_PREFIX to the exclusion filter in session_only_state.
Test: extend existing test_append_event_with_temp_state to assert temp
keys are absent from the persisted session state dict.

Merge #5841

Change-Id: I3278c205bba7c7e287308c2625112789593bb7ef

Author: Tinnitussen

src/google/adk/integrations/firestore/firestore_session_service.py

@@ -551,6 +551,7 @@ async def _append_txn(transaction: firestore.AsyncTransaction) -> int:
             for k, v in session.state.items()
             if not k.startswith(State.APP_PREFIX)
             and not k.startswith(State.USER_PREFIX)
+            and not k.startswith(State.TEMP_PREFIX)
         }
         transaction.update(
             session_ref,

tests/unittests/integrations/firestore/test_firestore_session_service.py

@@ -386,6 +386,15 @@ async def test_append_event_with_temp_state(mock_firestore_client):
   assert "temp:k1" not in event_data["actions"]["state_delta"]
   assert event_data["actions"]["state_delta"]["session_key"] == "session_val"
 
+  # 3. Verify temp keys are NOT written to session state in Firestore
+  transaction.update.assert_called_once()
+  update_args, _ = transaction.update.call_args
+  persisted_state = update_args[1]["state"]
+  assert (
+      "temp:k1" not in persisted_state
+  ), "temp: keys must not be persisted to Firestore session state"
+  assert "session_key" in persisted_state
+
 
 @pytest.mark.asyncio
 async def test_list_sessions_with_user_id(mock_firestore_client):