[vllm] fix: ensure AsyncLLM response_length less equal than max_new_tokens in generation_config.json

[Yangruipis]

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What does this PR do?

• max_tokens is deprecated for vllm openai endpoint, use max_completion_tokens instead.
• VLLM will use max_new_tokens in generation_config.json as the SERVER's max tokens (see https://github.com/vllm-project/vllm/pull/12242), even if config.response_length is greater than max_new_tokens, the model will still use config in generation_config.json, which may lead to unexpected behaviour(response always stop early). So i'll check when initialization and raise if exceed.

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• Issue Number: Fixes issue # or discussion # if any.
• Training: [Note which backend this PR will affect: FSDP, Megatron, both, or none]
• Inference: [Note which backend this PR will affect: vLLM, SGLang, both, or none]

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