When the bot sends a message in a thread, track its ts in _bot_message_ts.
When the bot is @mentioned in a thread, register it in _mentioned_threads.
Both sets enable auto-responding to future messages in those threads
without requiring repeated @mentions — making the bot behave like a
team member that stays engaged once a conversation starts.
Channel message gating now checks 4 signals (in order):
1. @mention in this message
2. Reply in a thread the bot started/participated in (_bot_message_ts)
3. Message in a thread where the bot was previously @mentioned (_mentioned_threads)
4. Existing session for this thread (_has_active_session_for_thread — survives restarts)
Thread context fetching now triggers on ANY first-entry path (not just
@mention), so the agent gets context whether it's entering via a mention,
a bot-thread reply, or a mentioned-thread auto-trigger.
Both tracking sets are bounded (5000 cap with prune-oldest-half) to prevent
unbounded memory growth in long-running deployments.
Salvaged from PR #5754 by @hhhonzik. Preserves our existing approval buttons,
thread context fetching, and session key fix. Does NOT include the
edit_message format_message() removal (that was a regression in the original PR).
Tests: 4 new tests for bot-ts tracking and mentioned-thread bounds.
e49c8bbbbb