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).

tests/gateway/test_web.py

@@ -72,7 +72,7 @@ class TestConfigEnvOverrides(unittest.TestCase):
 
     @patch.dict(os.environ, {
         "WEB_UI_ENABLED": "true",
-        "WEB_TOKEN": "",
+        "WEB_UI_TOKEN": "",
     }, clear=False)
     def test_web_defaults(self):
         config = GatewayConfig()