After a drain loop empties the queue it deletes the key from
FOLLOWUP_QUEUES. If a new message arrives at that moment
enqueueFollowupRun creates a fresh queue object with draining:false
but never starts a drain, leaving the message stranded until the
next run completes and calls finalizeWithFollowup.
Fix: persist the most recent runFollowup callback per queue key in
FOLLOWUP_RUN_CALLBACKS (drain.ts). enqueueFollowupRun now calls
kickFollowupDrainIfIdle after a successful push; if a cached
callback exists and no drain is running it calls scheduleFollowupDrain
to restart immediately. clearSessionQueues cleans up the callback
cache alongside the queue state.
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