fix: durable monkey-patch for Speechmatics finalize on flush

Move the SpeechStream._process_audio patch from container runtime
into our own source code so it survives Docker rebuilds. The patch
adds client.finalize() on FlushSentinel so EXTERNAL mode produces
final transcripts when LiveKit's VAD detects end of speech.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/services/voice-agent/src/plugins/speechmatics_stt.py

@@ -2,17 +2,54 @@
 Speechmatics STT factory for voice-agent.
 
 Creates a livekit-plugins-speechmatics STT instance configured for
-Mandarin-English bilingual recognition with speaker diarization support.
+Mandarin recognition with speaker diarization support.
 
 The SPEECHMATICS_API_KEY environment variable is read automatically
 by the livekit-plugins-speechmatics package.
 """
+import asyncio
 import logging
 
+from livekit.agents import stt, utils
 from livekit.plugins.speechmatics import STT, TurnDetectionMode
+from livekit.plugins.speechmatics.stt import SpeechStream
 
 logger = logging.getLogger(__name__)
 
+# ---------------------------------------------------------------------------
+# Monkey-patch SpeechStream._process_audio so that a FlushSentinel triggers
+# client.finalize().  In EXTERNAL turn-detection mode the Speechmatics server
+# only emits AddSegment (FINAL_TRANSCRIPT) after an explicit finalize signal.
+# The upstream plugin flushes the byte-stream but never calls finalize(),
+# which means LiveKit's VAD-driven flush never produces a final transcript.
+# ---------------------------------------------------------------------------
+_original_process_audio = SpeechStream._process_audio
+
+
+async def _patched_process_audio(self: SpeechStream) -> None:  # type: ignore[override]
+    try:
+        audio_bstream = utils.audio.AudioByteStream(
+            sample_rate=self._stt._sample_rate,
+            num_channels=1,
+        )
+        async for data in self._input_ch:
+            if isinstance(data, self._FlushSentinel):
+                frames = audio_bstream.flush()
+                # >>> patch: tell Speechmatics the turn ended <<<
+                if self._client and self._client._is_connected:
+                    self._client.finalize()
+            else:
+                frames = audio_bstream.write(data.data.tobytes())
+
+            if self._client:
+                for frame in frames:
+                    self._speech_duration += frame.duration
+                    await self._client.send_audio(frame.data.tobytes())
+    except asyncio.CancelledError:
+        pass
+
+
+SpeechStream._process_audio = _patched_process_audio  # type: ignore[assignment]
 
 # Map Whisper language codes to Speechmatics language codes
 _LANG_MAP = {