fix: harden upstream path + binary-body validation (gadfly round 1)

- upstreamPath rejects '..' in model ids AND in the rest path — the rest
  can embed SERVER-SUPPLIED components (ACE-Step result file URLs), so
  dot-dot/scheme smuggling toward other proxy endpoints is refused
- singleImageResult requires positive image evidence (sniffed magic OR
  declared image/*): an empty-Content-Type error page can no longer pass
  as 'the image' via sniffImageMIME's PNG-default labelling
- upscale/background responses get a dedicated 256MB cap (the 64MB cap
  is JSON-sized; a 4x PNG legitimately exceeds it)
- mesh JSON-detection widened (512-byte whitespace-tolerant peek + reject
  declared application/json)
- Transcribe now reuses buildMultipart; transcriptionFilename takes
  (filename, mime) so diarize shares it without a fake request struct;
  truncateForError stops shadowing builtin cap; OnlyMask doc de-ambiguated

Co-Authored-By: Claude Fable 5 <[email protected]>

provider/llamaswap/diarize.go

@@ -79,7 +79,7 @@ func (m *diarizationModel) Diarize(ctx context.Context, req audio.DiarizationReq
 	}
 
 	body, contentType, err := buildMultipart("build diarization form",
-		filePart{field: "audio_file", filename: diarizationFilename(req), data: req.Audio},
+		filePart{field: "audio_file", filename: transcriptionFilename(req.Filename, req.MIME), data: req.Audio},
 		nil)
 	if err != nil {
 		return nil, err
@@ -111,12 +111,3 @@ func (m *diarizationModel) Diarize(ctx context.Context, req audio.DiarizationReq
 	}
 	return res, nil
 }
-
-// diarizationFilename mirrors transcriptionFilename for the diarization
-// request shape (same untrusted-metadata sanitization rules).
-func diarizationFilename(req audio.DiarizationRequest) string {
-	return transcriptionFilename(audio.TranscriptionRequest{
-		Filename: req.Filename,
-		MIME:     req.MIME,
-	})
-}