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).
