fix(whisper): scrub invalid UTF-8 from segment text before protobuf marshal

whisper.cpp can emit bytes that are not valid UTF-8 — typically a
multibyte codepoint split across token boundaries. protobuf string
fields reject those at marshal time, which would surface as a transcribe
failure. Run strings.ToValidUTF8 on the segment text before it leaves
the cgo boundary so the bad byte gets replaced with U+FFFD.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Author: mudler

backend/go/whisper/gowhisper.go

@@ -139,7 +139,10 @@ func (w *Whisper) AudioTranscription(opts *pb.TranscriptRequest) (pb.TranscriptR
 		// segment start/end conversion factor taken from https://github.com/ggml-org/whisper.cpp/blob/master/examples/cli/cli.cpp#L895
 		s := CppGetSegmentStart(i) * (10000000)
 		t := CppGetSegmentEnd(i) * (10000000)
-		txt := strings.Clone(CppGetSegmentText(i))
+		// whisper.cpp can emit bytes that aren't valid UTF-8 (e.g. a multibyte
+		// codepoint split across token boundaries); protobuf string fields
+		// reject those at marshal time. Scrub before the value escapes cgo.
+		txt := strings.ToValidUTF8(strings.Clone(CppGetSegmentText(i)), "�")
 		tokens := make([]int32, CppNTokens(i))
 
 		if opts.Diarize && CppGetSegmentSpeakerTurnNext(i) {