fix: use EXTERNAL mode + patch plugin to finalize on flush

EXTERNAL mode produces partial transcripts but livekit-plugins-speechmatics
does not call finalize() when receiving a flush sentinel from the framework.
A runtime monkey-patch on the plugin's SpeechStream._process_audio adds the
missing finalize() call so final transcripts are generated.

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

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

@@ -297,18 +297,13 @@ async def entrypoint(ctx: JobContext) -> None:
             engine_type=engine_type,
         )
 
-        # Create and start AgentSession with the full pipeline.
-        # Speechmatics handles end-of-utterance natively via its Voice Agent
-        # API, so we use turn_detection="stt" to let it drive turn boundaries.
-        session_kwargs = dict(
+        # Create and start AgentSession with the full pipeline
+        session = AgentSession(
             vad=ctx.proc.userdata["vad"],
             stt=stt,
             llm=llm,
             tts=tts,
         )
-        if stt_provider == "speechmatics":
-            session_kwargs["turn_detection"] = "stt"
-        session = AgentSession(**session_kwargs)
 
         await session.start(
             agent=IT0VoiceAgent(),

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

@@ -9,7 +9,7 @@ by the livekit-plugins-speechmatics package.
 """
 import logging
 
-from livekit.plugins.speechmatics import STT
+from livekit.plugins.speechmatics import STT, TurnDetectionMode
 
 logger = logging.getLogger(__name__)
 
@@ -39,6 +39,7 @@ def create_speechmatics_stt(language: str = "cmn") -> STT:
     stt = STT(
         language=sm_lang,
         include_partials=True,
+        turn_detection_mode=TurnDetectionMode.EXTERNAL,
     )
     # Workaround: LiveKit's LanguageCode normalizes ISO 639-3 "cmn" back to
     # ISO 639-1 "zh", but Speechmatics expects "cmn".  Override the internal