fix: replaced hard coded value for user_id to the value from the tool context from parent agent.

Author: akshaypachpute-1998

src/google/adk/tools/agent_tool.py

@@ -136,7 +136,7 @@ async def run_async(
     )
     session = await runner.session_service.create_session(
         app_name=self.agent.name,
-        user_id='tmp_user',
+        user_id=tool_context._invocation_context.user_id,
         state=tool_context.state.to_dict(),
     )