fix(kokoros): implement audio_transcription_stream trait stub (#9422)

The backend.proto was updated to add AudioTranscriptionStream RPC, but
the Rust KokorosService was never updated to match the regenerated
tonic trait, breaking compilation with E0046.

Stubs the new streaming method as unimplemented, matching the pattern
used for the other streaming RPCs Kokoros does not support.

Author: mudler

backend/rust/kokoros/src/service.rs

@@ -341,6 +341,16 @@ impl Backend for KokorosService {
         Err(Status::unimplemented("Not supported"))
     }
 
+    type AudioTranscriptionStreamStream =
+        ReceiverStream<Result<backend::TranscriptStreamResponse, Status>>;
+
+    async fn audio_transcription_stream(
+        &self,
+        _: Request<backend::TranscriptRequest>,
+    ) -> Result<Response<Self::AudioTranscriptionStreamStream>, Status> {
+        Err(Status::unimplemented("Not supported"))
+    }
+
     async fn sound_generation(
         &self,
         _: Request<backend::SoundGenerationRequest>,