ci: harden release workflow and fix flaky test

tests/unit/test_load_control.py

@@ -144,41 +144,48 @@ def test_context_manager_exception_handling(self):
         assert controller.queue_depth == 0
 
     def test_concurrent_operations(self):
-        """Test controller under high concurrency."""
-        controller = BackpressureController(max_concurrent=5, queue_size=10, timeout=0.1)
+        """Test controller under high concurrency.
+
+        Uses a Barrier to force all threads to compete simultaneously,
+        and work duration >> timeout so permits can't be recycled.
+        Capacity = max_concurrent(2) + queue_size(3) = 5, so 15 of 20
+        threads must be rejected — no timing sensitivity.
+        """
+        controller = BackpressureController(max_concurrent=2, queue_size=3, timeout=0.01)
 
         results = []
         exceptions = []
+        barrier = threading.Barrier(20, timeout=5)
 
         def worker(worker_id):
             from cachekit.backends.errors import BackendError
 
             try:
+                barrier.wait()  # All threads launch simultaneously
                 with controller.acquire():
-                    time.sleep(0.05)  # Simulate work
+                    time.sleep(0.2)  # Hold permit far longer than timeout
                     results.append(worker_id)
             except BackendError as e:
                 exceptions.append((worker_id, str(e)))
+            except threading.BrokenBarrierError:
+                exceptions.append((worker_id, "barrier broken"))
 
-        # Launch many concurrent workers
         threads = []
-        for i in range(20):  # More than max_concurrent + queue_size
+        for i in range(20):
             t = threading.Thread(target=worker, args=(i,))
             threads.append(t)
             t.start()
 
-        # Wait for all threads
         for t in threads:
             t.join()
 
-        # Some should succeed, some should be rejected
         assert len(results) > 0, "Some operations should succeed"
         assert len(exceptions) > 0, "Some operations should be rejected"
         assert len(results) + len(exceptions) == 20
 
         # Verify final state is clean
         assert controller.queue_depth == 0
-        assert controller._semaphore._value == 5  # Back to max_concurrent
+        assert controller._semaphore._value == 2  # Back to max_concurrent
 
     def test_metrics_tracking(self):
         """Test that metrics are properly tracked."""