fix: preserve text task context in OpenAI chat adapter

[Genmin]

Summary

• preserve the constructed ctx for text-only OpenAI chat requests instead of sending only the raw doc_to_messages question text
• keep explicit multi-message chats and multimodal messages on the existing doc_to_messages path
• add regression coverage for text-only replacement plus multimodal/chat-native preservation

Fixes #1319.

Validation

• uv run --with pytest --with loguru --with python-dotenv --with openai --with pydantic --with tqdm --with numpy --with pillow --with accelerate python -m pytest test/models/test_openai_chat.py -q
• uv run --with loguru --with python-dotenv --with openai --with pydantic --with tqdm --with numpy --with pillow --with accelerate python -m py_compile lmms_eval/models/chat/openai.py test/models/test_openai_chat.py
• git diff --check

[kcz358]

Hi, I think this would hurt most of the tasks that use doc to messages to construct the messages. If there are tasks with wrong auto wrapped doc to messages function, the tasks should be revise instead of changing the doc to messages logic in the oai model.

[Genmin]

Thanks for taking a look. I agree with the concern: this is too broad for the model adapter layer, and task-specific message construction should be fixed at the task level instead. Closing this to avoid pushing behavior that could regress normal doc_to_messages tasks.