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

@@ -7,6 +7,11 @@
 #include <queue>
 #include <condition_variable>
 
+#ifdef ARCANA_TESTING_HOOKS
+#include <functional>
+inline std::function<void()> g_beforeWaitCallback{[]() {}};
+#endif
+
 namespace arcana
 {
     template<typename T, size_t max_size = std::numeric_limits<size_t>::max()>
@@ -95,6 +100,9 @@ namespace arcana
             {
                 while (!cancel.cancelled() && m_data.empty())
                 {
+#ifdef ARCANA_TESTING_HOOKS
+                    g_beforeWaitCallback();
+#endif
                     m_dataReady.wait(lock);
                 }
             }
@@ -116,6 +124,9 @@ namespace arcana
             {
                 while (!cancel.cancelled() && m_data.empty())
                 {
+#ifdef ARCANA_TESTING_HOOKS
+                    g_beforeWaitCallback();
+#endif
                     m_dataReady.wait(lock);
                 }
             }