[Feature]: `kv_transfer_params` not returned for multiple subrequests

[jianzs]

🚀 The feature, motivation and pitch

Currently, when handling HTTP requests with multiple subrequests, the response only includes kv_transfer_params for one subrequest, making it impossible to access KV transfer information for other subrequests.

Related PR: #17751 Reference code: https://github.com/vllm-project/vllm/blob/e384f2f10824df7789c6da35256cf957788c0208/vllm/entrypoints/openai/serving_completion.py#L514-L520

Alternatives

No response

Additional context

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