fix: correct Speechmatics STT language mapping and parameter name

- Map Whisper language codes (zh→cmn, en→en, etc.) to Speechmatics codes
- Fix parameter name: enable_partials → include_partials

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

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

@@ -14,19 +14,31 @@ from livekit.plugins import speechmatics
 logger = logging.getLogger(__name__)
 
 
+# Map Whisper language codes to Speechmatics language codes
+_LANG_MAP = {
+    "zh": "cmn",
+    "en": "en",
+    "ja": "ja",
+    "ko": "ko",
+    "de": "de",
+    "fr": "fr",
+}
+
+
 def create_speechmatics_stt(language: str = "cmn") -> speechmatics.STT:
     """Create a Speechmatics STT instance for the voice pipeline.
 
     Args:
-        language: Speechmatics language code. Default 'cmn' for Mandarin Chinese.
-                  Use 'cmn_en' for Mandarin-English bilingual, 'en' for English.
+        language: Language code (Whisper or Speechmatics). Whisper codes like
+                  'zh' are automatically mapped to Speechmatics equivalents.
 
     Returns:
         Configured speechmatics.STT instance.
     """
+    sm_lang = _LANG_MAP.get(language, language)
     stt = speechmatics.STT(
-        language=language,
-        enable_partials=True,
+        language=sm_lang,
+        include_partials=True,
     )
-    logger.info("Speechmatics STT created: language=%s", language)
+    logger.info("Speechmatics STT created: language=%s (input=%s)", sm_lang, language)
     return stt