fix: standardize storage update marker to string

Author: Dumeng

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

@@ -338,9 +338,7 @@ async def get_session(
         events=events,
         last_update_time=last_update_time,
     )
-    session._storage_update_marker = (
-        str(current_revision) if current_revision > 0 else None
-    )
+    session._storage_update_marker = str(current_revision)
     return session
 
   async def list_sessions(

tests/unittests/integrations/firestore/test_firestore_session_service.py

@@ -86,6 +86,7 @@ async def test_create_session(mock_firestore_client):
   assert session.app_name == app_name
   assert session.user_id == user_id
   assert session.id
+  assert session._storage_update_marker == "0"
 
   mock_firestore_client.collection.assert_any_call("adk-session")
   mock_firestore_client.collection.assert_any_call("app_states")
@@ -194,6 +195,7 @@ async def test_get_session_found(mock_firestore_client):
   assert session.state == {"key": "value"}
   assert len(session.events) == 1
   assert session.events[0].invocation_id == "test_inv"
+  assert session._storage_update_marker == "0"
 
 
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