fix(whisperx): keep the transcript when diarization fails, report real errors (#12427)

AudioTranscription caught every exception and returned an empty
TranscriptResult. A failed diarization step therefore discarded a
transcript that was already finished: with an HF token that has not
accepted the terms of the gated pyannote pipeline, the download fails
with 403 and every transcription came back as an empty text with
HTTP 200.

Diarization now degrades: if it fails, the transcript is returned
without speaker labels and the reason is logged. Any other failure
aborts the call with INTERNAL instead of pretending success with an
empty text.

Assisted-by: Claude:claude-opus-5-5

Signed-off-by: Stefan Walcz <stefan.walcz@walcz.de>

docs/content/features/audio-to-text.md

@@ -112,6 +112,8 @@ In addition to `file` and `model`, the endpoint accepts the following multipart
 | `stream` | When `true`, the endpoint emits an SSE stream of `transcript.text.delta` events followed by a final `transcript.text.done` event. |
 | `diarize` | LocalAI extension - speaker diarization. WhisperX requires `HF_TOKEN`; requests fail with `FailedPrecondition` when it is missing. |
 
+If speaker diarization fails after transcription succeeded, the WhisperX backend logs the error and returns the transcript without speaker labels. Other transcription failures return an error instead of an empty transcript. Diarization still requires `HF_TOKEN`.
+
 The response body for `verbose_json` includes `text`, `language`, `duration`, and `segments[]` (with `speaker` populated when diarization is enabled).
 
 ## Streaming transcriptions