[Feature]: Rename ar_stop_token_ids to a more descriptive name

🚀 The feature, motivation and pitch

The variable ar_stop_token_ids in vllm_omni/entrypoints/openai/serving_chat.py:2296 has an awkward name. It was introduced in PR #3896 and noted during code review (https://github.com/vllm-project/vllm-omni/pull/3896#discussion_r3328486645).

The name ar_stop_token_ids should be renamed to something more descriptive and idiomatic to improve code readability.

Alternatives

• ar_stop_token_ids → autoregressive_stop_token_ids (more explicit)
• ar_stop_token_ids → stop_token_ids_for_ar_stage (more descriptive of the context)
• ar_stop_token_ids → ar_stage_stop_token_ids (alternative ordering)

Additional context

This variable is used in _build_multistage_generation_inputs to store stop token IDs resolved for the AR (autoregressive/LLM) stage. It is later assigned to default_stage_params.stop_token_ids when stage_type == "llm". A clearer name would help future readers.

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