Fix kokoros backend build break from Backend trait drift (#9972)

* Initial plan

* fix(kokoros): implement missing AudioToAudioStream trait stubs

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

Author: Copilot

backend/rust/kokoros/src/service.rs

@@ -375,6 +375,15 @@ impl Backend for KokorosService {
         Err(Status::unimplemented("Not supported"))
     }
 
+    type AudioToAudioStreamStream = ReceiverStream<Result<backend::AudioToAudioResponse, Status>>;
+
+    async fn audio_to_audio_stream(
+        &self,
+        _: Request<tonic::Streaming<backend::AudioToAudioRequest>>,
+    ) -> Result<Response<Self::AudioToAudioStreamStream>, Status> {
+        Err(Status::unimplemented("Not supported"))
+    }
+
     async fn sound_generation(
         &self,
         _: Request<backend::SoundGenerationRequest>,