fix: accept **kwargs in send_voice for Discord and Slack adapters

play_tts base class forwards metadata via **kwargs to send_voice,
but Discord and Slack adapters did not accept extra keyword arguments,
causing TypeError and silent message handling failure.

Also fix test_web_defaults to patch correct env var (WEB_UI_TOKEN).

gateway/platforms/slack.py

@@ -506,6 +506,7 @@ class SlackAdapter(BasePlatformAdapter):
         caption: Optional[str] = None,
         reply_to: Optional[str] = None,
         metadata: Optional[Dict[str, Any]] = None,
+        **kwargs,
     ) -> SendResult:
         """Send an audio file to Slack."""
         try: