fix: add explicit metadata param to Discord send_voice signature

gateway/platforms/discord.py

@@ -646,6 +646,7 @@ class DiscordAdapter(BasePlatformAdapter):
         audio_path: str,
         caption: Optional[str] = None,
         reply_to: Optional[str] = None,
+        metadata: Optional[Dict[str, Any]] = None,
         **kwargs,
     ) -> SendResult:
         """Send audio as a Discord file attachment."""