A reconnect can replace the delivery adapter while the busy handler awaits
its acknowledgment. Resolving only the current delivery adapter then misses
a follow-up already accepted into the old adapter's slot, leaving it with
no guard or task. Check this adapter's slot and an idle replacement's, and
start the event on the adapter whose slot held it.
Replaces the stubbed replacement-slot test with a real busy-path test that
swaps the adapter before and after enqueue.
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