[serve.llm][Fix] retry with model_config arg

[lk-chen]

Why are these changes needed?

vLLM changed (non-external-facing) api in chat_utils.py, we need to adapt the argument change. see vllm-project/vllm#18098

Related issue number

FIX #52975

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[GeneDer]

Some nits and suggestions, but thanks for fixing it!

[GeneDer]

Thanks for addressing the comments!