move session creation to runner level

Author: lwangverizon

src/google/adk/runners.py

@@ -454,8 +454,9 @@ async def _run_with_trace(
             app_name=self.app_name, user_id=user_id, session_id=session_id
         )
         if not session:
-          message = self._format_session_not_found_message(session_id)
-          raise ValueError(message)
+          session = await self.session_service.create_session(
+              app_name=self.app_name, user_id=user_id, session_id=session_id
+          )
         if not invocation_id and not new_message:
           raise ValueError(
               'Running an agent requires either a new_message or an '

src/google/adk/sessions/database_session_service.py

@@ -436,8 +436,8 @@ async def append_event(self, session: Session, event: Event) -> Event:
       if storage_session.update_timestamp_tz > session.last_update_time:
         raise ValueError(
             "The last_update_time provided in the session object"
-            f" {datetime.fromtimestamp(session.last_update_time):'%Y-%m-%d %H:%M:%S'} is"
-            " earlier than the update_time in the storage_session"
+            f" {datetime.fromtimestamp(session.last_update_time):'%Y-%m-%d %H:%M:%S'}"
+            " is earlier than the update_time in the storage_session"
             f" {datetime.fromtimestamp(storage_session.update_timestamp_tz):'%Y-%m-%d %H:%M:%S'}."
             " Please check if it is a stale session."
         )