fix: add agent_id field to GCS PubSub spend_logs_payload.json test expectation (#17938)

- Add agent_id: null to expected JSON to match actual payload structure
- Fixes test_async_gcs_pub_sub_v1 test failure
- agent_id is an optional field in SpendLogsPayload that is always included (as null when not provided)

tests/logging_callback_tests/gcs_pub_sub_body/spend_logs_payload.json

@@ -28,5 +28,6 @@
     "response": "{}",
     "proxy_server_request": "{}",
     "status": "success",
-    "mcp_namespaced_tool_name": null
+    "mcp_namespaced_tool_name": null,
+    "agent_id": null
 }