fix(core): convert AsyncOperation to UniTask for Unity 6 compatibility

In Unity 6, UnityEngine.AsyncOperation cannot be directly awaited.
Convert to UniTask using .ToUniTask() extension method.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: JasonXuDeveloper - 傑 <jason@xgamedev.net>

Author: JasonXuDeveloper

UnityProject/Packages/com.jasonxudeveloper.jengine.core/Runtime/Bootstrap.Common.cs

@@ -92,7 +92,7 @@ public static async UniTask<SceneHandle> LoadHotUpdateScene(ResourcePackage pack
                 // Call error callback
                 await callbacks.OnError(e);
                 // Switch back to previous scene
-                await SceneManager.LoadSceneAsync(previousSceneName);
+                await SceneManager.LoadSceneAsync(previousSceneName).ToUniTask();
 
                 return null;
             }