### Note on root cause

The status-code mechanism below is **verified by reading the code** on `upstream/main`. The concrete trigger (an Azure `gpt-audio` deployment rejecting the speech operation) is **verified from live proxy tracebacks and access logs**. The generalization to "all client-side errors on this route" is **inferred** from the code path; I directly observed only the `BadRequestError` / `OperationNotSupported` case.

### Check for existing issues

- [x] I have searched the existing issues

Searched several angles (`audio/speech 500`, `gpt-audio OperationNotSupported`, `aspeech status code`, `tts 500 instead of 400`, `audio speech BadRequest`); no existing issue or PR covers this. The `stream_chunk_builder` 500 on `/v1/messages` (#32951) is unrelated (different route and root cause).

### What happened

A `POST /v1/audio/speech` request whose underlying provider call fails with a client error (HTTP 4xx) is surfaced to the caller as **HTTP 500** with a generic body:

```json {"error":{"message":"Internal server error","type":"internal_server_error"}} ```

The same underlying error on `POST /v1/audio/transcriptions` is correctly surfaced as **HTTP 400** with the real pro…