Added validity check to task handles and fixed some clang-tidy warning in MIJIN_ASSERT.

source/mijin/async/coroutine.hpp

@@ -416,6 +416,11 @@ public:
     TaskHandle& operator=(const TaskHandle&) = default;
     TaskHandle& operator=(TaskHandle&&) = default;
 
+    [[nodiscard]] bool isValid() const noexcept
+    {
+        return !state_.expired();
+    }
+
     void cancel() const noexcept
     {
         if (std::shared_ptr<TaskSharedState> state = state_.lock())