Cross-PR interaction fix: #77714 (salvage of #71775) changed
_available_entries to return (available, pending_refresh) while #77631
(salvage of #67642) added next_available_at() which still truthiness-
tests the bare return. A non-empty tuple is always truthy — even
([], []) — so the reset-aware gate silently returned None ('no wait
info') for every exhausted pool, disabling the feature #77631 shipped.
Unpack the tuple and test the available list.
Also adapts the lock-probe test for the RLock introduced by #77714
(same-thread non-blocking acquire always succeeds on an RLock; probe
from a helper thread instead).
