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:

tests/run_agent/test_reset_aware_primary_restore.py

@@ -203,9 +203,23 @@ class TestNextAvailableAt:
         original = pool._available_entries
 
         def _probe(**kwargs):
-            held["locked"] = not pool._lock.acquire(blocking=False)
-            if not held["locked"]:
-                pool._lock.release()
+            # self._lock is an RLock (deferred-refresh mutations self-lock),
+            # so a same-thread non-blocking acquire always succeeds; probe
+            # ownership from a helper thread instead.
+            import threading as _t
+
+            blocked = _t.Event()
+
+            def _try():
+                if not pool._lock.acquire(blocking=False):
+                    blocked.set()
+                else:
+                    pool._lock.release()
+
+            worker = _t.Thread(target=_try)
+            worker.start()
+            worker.join(timeout=5)
+            held["locked"] = blocked.is_set()
             return original(**kwargs)
 
         pool._available_entries = _probe