Implement DashScope ASR provider and enhance ASR service architecture

- Added DashScope ASR service implementation for real-time streaming.
- Updated ASR provider logic to support DashScope alongside existing providers.
- Enhanced runtime metadata resolution to include DashScope as a valid ASR provider.
- Modified configuration files and documentation to reflect the addition of DashScope.
- Introduced tests to validate DashScope integration and ASR service behavior.
- Refactored ASR service factory to accommodate new provider options and modes.

engine/providers/asr/buffered.py

@@ -34,6 +34,7 @@ class BufferedASRService(BaseASRService):
         language: str = "en"
     ):
         super().__init__(sample_rate=sample_rate, language=language)
+        self.mode = "offline"
         
         self._audio_buffer: bytes = b""
         self._current_text: str = ""
@@ -86,6 +87,23 @@ class BufferedASRService(BaseASRService):
         self._current_text = ""
         self._audio_buffer = b""
         return text
+
+    async def get_final_transcription(self) -> str:
+        """Offline compatibility method used by DuplexPipeline."""
+        return self.get_and_clear_text()
+
+    def clear_buffer(self) -> None:
+        """Offline compatibility method used by DuplexPipeline."""
+        self._audio_buffer = b""
+        self._current_text = ""
+
+    async def start_interim_transcription(self) -> None:
+        """No-op for plain buffered ASR."""
+        return None
+
+    async def stop_interim_transcription(self) -> None:
+        """No-op for plain buffered ASR."""
+        return None
     
     def get_audio_buffer(self) -> bytes:
         """Get accumulated audio buffer."""
@@ -103,6 +121,7 @@ class MockASRService(BaseASRService):
     
     def __init__(self, sample_rate: int = 16000, language: str = "en"):
         super().__init__(sample_rate=sample_rate, language=language)
+        self.mode = "offline"
         self._transcript_queue: asyncio.Queue[ASRResult] = asyncio.Queue()
         self._mock_texts = [
             "Hello, how are you?",
@@ -145,3 +164,18 @@ class MockASRService(BaseASRService):
                 continue
             except asyncio.CancelledError:
                 break
+
+    def clear_buffer(self) -> None:
+        return None
+
+    async def get_final_transcription(self) -> str:
+        return ""
+
+    def get_and_clear_text(self) -> str:
+        return ""
+
+    async def start_interim_transcription(self) -> None:
+        return None
+
+    async def stop_interim_transcription(self) -> None:
+        return None