fix: add missing choices/Choice to discord mock in test_discord_free_response

The mock's app_commands SimpleNamespace lacked choices and Choice attrs,
causing xdist test ordering failures when this mock loaded before
test_discord_slash_commands.

tests/gateway/test_discord_free_response.py

@@ -29,6 +29,8 @@ def _ensure_discord_mock():
     discord_mod.Embed = MagicMock
     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()