Port of the Slack half of #13855 (by @kshitijk4poor), reimplemented against
the plugin adapter (the original PR targets the deleted
gateway/platforms/slack.py and six other legacy adapters).
Channels listed in slack.require_mention_channels (config.yaml) or
SLACK_REQUIRE_MENTION_CHANNELS ALWAYS require an explicit @mention, even
when require_mention is false globally or the channel is in
free_response_channels — the opposite direction of free_response_channels.
Instead of duplicating the PR's inline reply-to-bot-thread/mentioned-thread/
session checks, the forced channel falls through to the SAME decision chain
as normal mention gating, so all five wake checks in
_should_wake_on_unmentioned_message keep applying (single decision path).
Credit: adapted from #13855 by @kshitijk4poor (Slack half only; the other
platform halves target deleted legacy adapters and are out of scope for
this cluster).
Teknium
b416907538