Add provider info to meta.extra in streaming responses

[devin-ai-integration]

Summary

Injects the upstream STT provider name (e.g. "deepgram", "soniox") into metadata.extra.provider on every TranscriptResponse returned through the hyprnote response transformer in transcribe-proxy.

Two small changes:

• owhisper-interface: adds StreamResponse::set_provider(&mut self, provider: &str) — inserts/upserts a "provider" key into metadata.extra
• transcribe-proxy: calls set_provider inside build_response_transformer after parsing each upstream response

Review & Testing Checklist for Human

• Passthrough path coverage: set_provider is only called in the hyprnote response-transformer path. The passthrough path (passthrough.rs) forwards raw upstream JSON and will not include provider in metadata.extra. Verify this is acceptable for your use case.
• Provider name format: The name is derived via format!("{:?}", provider).to_lowercase(). Confirm the Debug impl on Provider produces the expected strings for all variants (deepgram, soniox, assemblyai, etc.).
• Downstream consumers: Confirm that clients parsing metadata.extra handle the new "provider" key gracefully (it's additive, but worth a quick check).

Notes

Requested by: @yujonglee
Link to Devin run

[devin-ai-integration]

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