A channel turn can outlast the idle interval, leaving its pool eligible
for teardown immediately after completion. Reset the idle clock when a
channel result settles, while letting heartbeat-only pools expire
normally. Add regression coverage for both cases.
This addresses premature idle teardown. Broader callback continuity and
human workflow testing remain pending, so this PR stays in draft.
Signed-off-by: Benoit Quenaudon <bquenaudon@squareup.com>
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