fix: Refactor session retrieval fallback _streaming_agent_run_with_events.

PiperOrigin-RevId: 868781249

Author: Tongzhou-Jiang

vertexai/agent_engines/templates/adk.py

@@ -1198,6 +1198,13 @@ async def streaming_agent_run_with_events(self, request_json: str):
                     artifact_service=artifact_service,
                     request=request,
                 )
+            if not session:
+                #  Another fall back for get_session that doesn't raise.
+                session = await self._init_session(
+                    session_service=session_service,
+                    artifact_service=artifact_service,
+                    request=request,
+                )
         else:
             # Not providing a session ID will create a new in-memory session.
             session_service = self._tmpl_attrs.get("in_memory_session_service")

vertexai/preview/reasoning_engines/templates/adk.py

@@ -1030,6 +1030,13 @@ async def _invoke_agent_async():
                         artifact_service=artifact_service,
                         request=request,
                     )
+                if not session:
+                    #  Another fall back for get_session that doesn't raise.
+                    session = await self._init_session(
+                        session_service=session_service,
+                        artifact_service=artifact_service,
+                        request=request,
+                    )
             else:
                 # Not providing a session ID will create a new in-memory session.
                 session_service = self._tmpl_attrs.get("in_memory_session_service")