Follow-up to the allowed_channels wildcard fix in the preceding commit.
The same '*' literal trap affected two other Discord channel config lists:
- DISCORD_IGNORED_CHANNELS: '*' was stored as the literal string in the
ignored set, and the intersection check never matched real channel IDs,
so '*' was a no-op instead of silencing every channel.
- DISCORD_FREE_RESPONSE_CHANNELS: same shape — '*' never matched, so
the bot still required a mention everywhere.
Add a '*' short-circuit to both checks, matching the allowed_channels
semantics. Extend tests/gateway/test_discord_allowed_channels.py with
regression coverage for all three lists.
Refs: #14920
8a1e247c6c