fix: time logging format (#1281)

Summary:
missed in last PR

Test Plan:
```
LLAMA_STACK_CONFIG=fireworks pytest -s -v tests/client-sdk/agents/test_agents.py::test_create_turn_response --safety-shield meta-llama/Llama-Guard-3-8B
```

llama_stack/providers/inline/agents/meta_reference/agent_instance.py

@@ -746,7 +746,7 @@ class ChatAgent(ShieldRunnerMixin):
                                     )
                                 ],
                                 started_at=tool_execution_start_time,
-                                completed_at=datetime.now(),
+                                completed_at=datetime.now().astimezone().isoformat(),
                             ),
                         )
                     )