fix: Add check for empty delta in OpenAI legacy chat

[badbye]

Kimi-cli raises an error when using Minimax:

kimi-cli/lib/python3.13/site-packages/kosong/chat_provider/openai_legacy.py", line 168, in _convert_stream_response
    if delta.content:
       ^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'content'

Because MiniMax-M2 returns Choice(delta=None, finish_reason='stop', index=0, logprobs=None, message={'content': '', 'role': 'assistant', 'name': 'MiniMax AI', 'audio_content': '', 'reasoning_content': ''}) in the end.

[stdrc]

Because MiniMax-M2 returns Choice(delta=None, finish_reason='stop', index=0, logprobs=None, message={'content': '', 'role': 'assistant', 'name': 'MiniMax AI', 'audio_content': '', 'reasoning_content': ''}) in the end.

MiniMax should fix their API to be OpenAI-compatible🤣. delta is not possible to be None according to OpenAI SDK:

[stdrc]

Closing if not further comment

[stdrc]

OK I changed my mind. We are planning to make OpenAILegacy chat provider a general OpenAI-compatible API provider, which means, we now accept special logic for specific platforms that are slightly incompatible with OpenAI.

[stdrc]

If you are still interested, please resolve the conflict and test locally again :). Now OpenAILegacy has been moved to kosong/contrib/chat_provider.

[pvzheroes125]

@badbye I think that it's the fault of deprecated endpoint https://api.minimaxi.com/v1/text/chatcompletion_v2, this endpoint will append a non-stream event without delta field in stream response, which is not standard behavior. To avoid this problem, please refer to https://platform.minimaxi.com/docs/api-reference/text-openai-api and use the new openai-compatible openai api