fix(credential_pool): unpack the tuple in next_available_at's gate

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).

agent/credential_pool.py

@@ -642,7 +642,8 @@ class CredentialPool:
         ``_available_entries`` caller (see the comment on ``has_available``).
         """
         with self._lock:
-            if self._available_entries():
+            available, _pending = self._available_entries()
+            if available:
                 return None
             candidates: List[float] = []
             for entry in self._entries: