fix: add choices/Choice to discord mock for /voice slash command test

tests/gateway/test_discord_slash_commands.py

@@ -21,6 +21,8 @@ def _ensure_discord_mock():
     discord_mod.Interaction = object
     discord_mod.app_commands = SimpleNamespace(
         describe=lambda **kwargs: (lambda fn: fn),
+        choices=lambda **kwargs: (lambda fn: fn),
+        Choice=lambda **kwargs: SimpleNamespace(**kwargs),
     )
 
     ext_mod = MagicMock()