fix(discord): extend role-scope fix to slash surface + fixture update

Sibling-site fix: _evaluate_slash_authorization was the fourth
_is_allowed_user caller and didn't pass guild/is_dm through, so slash
interactions would take the DM branch regardless of whether they came
from a guild channel. Now reads interaction.guild + in_dm and forwards.

Also updates test_discord_slash_auth fixture (_make_interaction) so
the SimpleNamespace guild mock has a get_member(uid)->None method —
required by the new guild-scoped fallback path in _is_allowed_user.
Tests exercising positive role paths still work via user.roles.

Three new regression tests in test_discord_roles_dm_scope:
- Slash DM + role in mutual public guild → rejected
- Slash in guild B + role only in guild A → rejected
- Slash in guild B + role in guild B → allowed (positive control)

368 Discord tests pass. test_discord_free_channel_skips_auto_thread
also fails on clean main (pre-existing, unrelated to this fix).

gateway/platforms/discord.py

@@ -2305,7 +2305,16 @@ class DiscordAdapter(BasePlatformAdapter):
             return (True, None)
 
         user_id = str(user.id)
-        if not self._is_allowed_user(user_id, author=user):
+        # Pass guild + is_dm so role check is scoped to the originating
+        # guild and cross-guild DM bypass (#12136) can't land via the
+        # slash surface either.
+        interaction_guild = getattr(interaction, "guild", None)
+        if not self._is_allowed_user(
+            user_id,
+            author=user,
+            guild=interaction_guild,
+            is_dm=in_dm,
+        ):
             return (
                 False,
                 "user not in DISCORD_ALLOWED_USERS / DISCORD_ALLOWED_ROLES",