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).

tests/gateway/test_discord_roles_dm_scope.py

@@ -252,3 +252,93 @@ def test_empty_allowlists_allow_everyone():
         adapter._is_allowed_user("42", author=None, guild=None, is_dm=True)
         is True
     )
+
+
+# ---------------------------------------------------------------------------
+# Slash-surface sibling site: _evaluate_slash_authorization must pass
+# guild/is_dm through so the cross-guild bypass can't land via slash either.
+# ---------------------------------------------------------------------------
+
+
+def test_slash_authorization_rejects_cross_guild_role_dm(monkeypatch):
+    """Slash interaction in a DM must not be authorized by a role held in
+    any mutual guild (parallel to the on_message cross-guild bypass)."""
+    import discord as _discord  # type: ignore
+    monkeypatch.delenv("DISCORD_DM_ROLE_AUTH_GUILD", raising=False)
+
+    public_guild, _ = _guild_with_member(
+        guild_id=111111,
+        member_id=42,
+        role_ids=[5555],
+    )
+    adapter = _make_adapter(
+        allowed_roles=[5555],
+        guilds=[public_guild],
+    )
+
+    # Fake a DM interaction: user is Member-like, channel is DMChannel,
+    # interaction.guild is None.
+    interaction = SimpleNamespace(
+        user=SimpleNamespace(id=42),
+        channel=MagicMock(spec=_discord.DMChannel),
+        channel_id=None,
+        guild=None,
+    )
+
+    allowed, reason = adapter._evaluate_slash_authorization(interaction)
+    assert allowed is False
+    assert "ALLOWED" in (reason or "")
+
+
+def test_slash_authorization_rejects_cross_guild_role_in_guild(monkeypatch):
+    """Slash in guild B must not be authorized by a role held in guild A."""
+    monkeypatch.delenv("DISCORD_DM_ROLE_AUTH_GUILD", raising=False)
+
+    public_guild, _ = _guild_with_member(
+        guild_id=111111,
+        member_id=42,
+        role_ids=[5555],
+    )
+    # Interaction arrives in trusted_guild where user 42 has no role
+    trusted_guild = SimpleNamespace(id=222222, get_member=lambda uid: None)
+    adapter = _make_adapter(
+        allowed_roles=[5555],
+        guilds=[public_guild, trusted_guild],
+    )
+
+    interaction = SimpleNamespace(
+        user=SimpleNamespace(id=42),
+        channel=SimpleNamespace(id=9999),  # not a DMChannel instance
+        channel_id=9999,
+        guild=trusted_guild,
+    )
+
+    allowed, reason = adapter._evaluate_slash_authorization(interaction)
+    assert allowed is False
+    assert "ALLOWED" in (reason or "")
+
+
+def test_slash_authorization_allows_in_scope_guild_role(monkeypatch):
+    """Positive control: slash in guild B, user has role in guild B → allowed."""
+    monkeypatch.delenv("DISCORD_DM_ROLE_AUTH_GUILD", raising=False)
+
+    trusted_guild, _ = _guild_with_member(
+        guild_id=222222,
+        member_id=42,
+        role_ids=[5555],
+    )
+    adapter = _make_adapter(
+        allowed_roles=[5555],
+        guilds=[trusted_guild],
+    )
+
+    interaction = SimpleNamespace(
+        user=SimpleNamespace(id=42),
+        channel=SimpleNamespace(id=9999),
+        channel_id=9999,
+        guild=trusted_guild,
+    )
+
+    allowed, reason = adapter._evaluate_slash_authorization(interaction)
+    assert allowed is True
+    assert reason is None

tests/gateway/test_discord_slash_auth.py

@@ -158,7 +158,11 @@ def _make_interaction(
 
     return SimpleNamespace(
         user=user_obj,
-        guild=SimpleNamespace(owner_id=999),
+        # `get_member` needed for the guild-scoped role fallback path in
+        # _is_allowed_user after the #12136 cross-guild fix. Fixture guild
+        # has no members by default — tests exercising positive role paths
+        # assign their own Member via user.roles + matching allowed_role_ids.
+        guild=SimpleNamespace(owner_id=999, id=guild_id, get_member=lambda uid: None),
         guild_id=guild_id,
         channel_id=channel_id,
         channel=channel,