Fix index out of range error when processing model output

[ldhldh]

Description of Changes

When using LLM like Qwen, an index out of range error occurs during the final output chunk because chunk.choices[0] may not exist. This PR adds a check for chunk.choices before accessing chunk.choices[0] to prevent this error.

Testing

• Tested with the Qwen model (agent_server.model = 'qwen-plus') using the following code snippet:

  async for chunk in agent_server.stream_response(message=message):
      print(chunk, end='', flush=True)