feat(voice): 语音silent降级信号(C2) — provider仅在结果为空时出现

- voice/Chain+Service: STTWithProvider/TTSWithProvider(真实档位cloud/local/edge-tts/espeak/silent), 旧方法改薄包装
- gateway: transcribe空文本带{text,provider} / voice_output空音频带provider / speech空音频tts_empty+X-Voice-Provider头
- middleware: CORS Access-Control-Expose-Headers: X-Voice-Provider
- 可选接口 voiceProviderReporter(type断言透传), 自定义实现回退silent
- 新增测试10个(provider_signal/voice_signal), build/vet/test全绿+交叉编译
- Docs/api-client v0.23 + api-reference CORS 同步

internal/runtime/multimodal/voice/service.go

@@ -53,9 +53,10 @@ type job struct {
 
 // jobResult carries the outcome written back to the caller.
 type jobResult struct {
-	text  string
-	audio Audio
-	err   error
+	text     string
+	audio    Audio
+	provider string
+	err      error
 }
 
 // NewService builds a Service backed by chain and starts the worker pool.
@@ -99,9 +100,9 @@ func (s *Service) run(j job) {
 	res := jobResult{}
 	switch j.kind {
 	case jobSTT:
-		res.text, res.err = s.chain.STT(s.bgCtx, j.audio)
+		res.text, res.provider, res.err = s.chain.STTWithProvider(s.bgCtx, j.audio)
 	case jobTTS:
-		res.audio, res.err = s.chain.TTS(s.bgCtx, j.text)
+		res.audio, res.provider, res.err = s.chain.TTSWithProvider(s.bgCtx, j.text)
 	}
 	j.res <- res
 }
@@ -109,21 +110,35 @@ func (s *Service) run(j job) {
 // STT enqueues a transcription request and waits for the result. ctx cancels
 // the wait and the underlying provider call.
 func (s *Service) STT(ctx context.Context, audio Audio) (string, error) {
+	text, _, err := s.STTWithProvider(ctx, audio)
+	return text, err
+}
+
+// STTWithProvider is STT but also reports the provider rung that produced the
+// transcription.
+func (s *Service) STTWithProvider(ctx context.Context, audio Audio) (string, string, error) {
 	res, err := s.submit(ctx, job{kind: jobSTT, audio: audio, res: make(chan jobResult, 1)})
 	if err != nil {
-		return "", err
+		return "", "", err
 	}
-	return res.text, res.err
+	return res.text, res.provider, res.err
 }
 
 // TTS enqueues a synthesis request and waits for the result. ctx cancels the
 // wait and the underlying provider call.
 func (s *Service) TTS(ctx context.Context, text string) (Audio, error) {
+	audio, _, err := s.TTSWithProvider(ctx, text)
+	return audio, err
+}
+
+// TTSWithProvider is TTS but also reports the provider rung that produced the
+// audio.
+func (s *Service) TTSWithProvider(ctx context.Context, text string) (Audio, string, error) {
 	res, err := s.submit(ctx, job{kind: jobTTS, text: text, res: make(chan jobResult, 1)})
 	if err != nil {
-		return Audio{}, err
+		return Audio{}, "", err
 	}
-	return res.audio, res.err
+	return res.audio, res.provider, res.err
 }
 
 // CloneVoice creates a custom voice, delegating to the underlying chain. It is a