fix: UnboundLocalError for role in streaming audio/image responses

[PierreLeGuen]

Summary

Fixes an UnboundLocalError when streaming audio or image responses. The role variable was only assigned inside the text-specific condition block, but was used unconditionally by audio and image handlers.

Problem

When making a streaming request with modalities: ["audio"], the server returns:

{"error": {"message": "cannot access local variable 'role' where it is not associated with a value", "type": "BadRequestError", "param": null, "code": 400}}

This happens because role is assigned only when final_output_type == "text" (line 648), but _create_audio_choice() and _create_image_choice() reference it regardless of output type.

Fix

Move role = self.get_chat_request_role(request) outside the text-specific condition block so it's initialized for all modalities, matching the non-streaming path behavior (line 1295).

Reproduction

curl -s http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "Qwen/Qwen3-Omni-30B-A3B-Instruct", "messages": [{"role": "user", "content": "Say hello"}], "modalities": ["audio"], "stream": true}'

[hsliuustc0106]

@GG-li PTAL

[hsliuustc0106]

@yenuo26 PTAL

[yenuo26]

I have verified that after adding this modification, #932 can be fixed.

[david6666666]

LGTM