Slack buffers un-acked Socket Mode events and replays them when the
websocket reconnects; the replay can arrive several minutes later —
past the 300s default dedup TTL — producing a duplicate bot reply.
Default the Slack dedup window to 1 hour (memory stays bounded by the
deduplicator's max_size LRU pruning) and allow overriding via
SLACK_DEDUP_TTL_SECONDS.
Salvaged from PR #40064 by @MrAbsaroka (reapplied onto the
plugin-migrated adapter path). Fixes#4777.
Matt Ferguson
3c5c389f18