chore: Improves session update time validation message (google#446)

Enhances the error message when a session's last update time is later than the storage update time.
This provides better readability by formatting the timestamps in the error message.

Co-authored-by: Hangfei Lin <hangfei@google.com>

Author: kakao-bart-lee

src/google/adk/sessions/database_session_service.py

@@ -484,9 +484,11 @@ def append_event(self, session: Session, event: Event) -> Event:
 
       if storage_session.update_time.timestamp() > session.last_update_time:
         raise ValueError(
-            f"Session last_update_time {session.last_update_time} is later than"
-            f" the upate_time in storage {storage_session.update_time}"
-        )
+          f"Session last_update_time "
+          f"{datetime.fromtimestamp(session.last_update_time):%Y-%m-%d %H:%M:%S} "
+          f"is later than the update_time in storage "
+          f"{storage_session.update_time:%Y-%m-%d %H:%M:%S}"
+      )
 
       # Fetch states from storage
       storage_app_state = sessionFactory.get(