[Bugfix][Router] Default transcription language to None (auto-detect)

[WaelRabah]

Summary

route_general_transcriptions was defaulting the transcription language to "en" when the client did not supply one. This forced the backend to transcribe in English even when the model could auto-detect the language.

vLLM's own default for language is None, which leaves language detection to the model. This change aligns the router with that behavior.

Changes

• src/vllm_router/services/request_service/request.py
  • Default language to None (form.get("language") instead of form.get("language", "en")).
  • Only include the language field in the outbound multipart form data when it is set. When absent, aiohttp.FormData would otherwise encode None as the literal string "None" and send it to the backend as an invalid language.
• src/tests/test_multipart_proxy.py
  • Update the audio transcription tests to assert the field is absent/None by default.

Testing

src/tests/test_multipart_proxy.py — 4 passed.

[gemini-code-assist]

Code Review

This pull request updates the transcription routing to stop defaulting the language parameter to 'en', allowing the backend to auto-detect the language when it is omitted. The corresponding test cases have been updated to reflect this change. Feedback suggests sanitizing the language input to filter out common null-like string values (such as 'none', 'null', or 'undefined') and whitespace, preventing potential backend validation errors from client-side serialization quirks.