Add optional testing hook for blocking_concurrent_queue

Add a global before-wait callback invoked while holding the queue mutex,
right before condition_variable::wait(). This enables deterministic testing
of lost-wakeup race conditions in code that uses the queue.

Behind #ifdef ARCANA_TESTING_HOOKS -- zero cost when not enabled.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bghgary

Source/Shared/arcana/threading/blocking_concurrent_queue.h

@@ -7,8 +7,24 @@
 #include <queue>
 #include <condition_variable>
 
+#ifdef ARCANA_TESTING_HOOKS
+#include <functional>
+#endif
+
 namespace arcana
 {
+#ifdef ARCANA_TESTING_HOOKS
+    namespace detail
+    {
+        inline std::function<void()> beforeWaitCallback{[]() {}};
+    }
+
+    inline void set_before_wait_callback(std::function<void()> callback)
+    {
+        detail::beforeWaitCallback = std::move(callback);
+    }
+#endif
+
     template<typename T, size_t max_size = std::numeric_limits<size_t>::max()>
     class blocking_concurrent_queue
     {
@@ -95,6 +111,9 @@ namespace arcana
             {
                 while (!cancel.cancelled() && m_data.empty())
                 {
+#ifdef ARCANA_TESTING_HOOKS
+                    detail::beforeWaitCallback();
+#endif
                     m_dataReady.wait(lock);
                 }
             }
@@ -116,6 +135,9 @@ namespace arcana
             {
                 while (!cancel.cancelled() && m_data.empty())
                 {
+#ifdef ARCANA_TESTING_HOOKS
+                    detail::beforeWaitCallback();
+#endif
                     m_dataReady.wait(lock);
                 }
             }