fix: fallback alert controller to UIScreen size (#53500)

Summary:
This PR falls back to UIScreen when windowScene is not available.

<img width="500" alt="CleanShot 2025-08-28 at 14 30 59@2x" src="https://github.com/user-attachments/assets/9dda3153-dfe7-48a5-9d0e-5416c2e34c64" />

## Changelog:

[IOS] [FIXED] - Simplify RCTAlertController, don't create additional UIWindow

Pull Request resolved: #53500

Test Plan:
Open the alert multiple times to check if everything works as expected.

Rollback Plan:

Reviewed By: javache

Differential Revision: D81410450

Pulled By: cipolleschi

fbshipit-source-id: c27ea98d9e811c2f259f0ff3c6689482d116c418

Author: okwasniewski

packages/react-native/React/CoreModules/RCTAlertController.mm

@@ -20,7 +20,12 @@ @implementation RCTAlertController
 - (UIWindow *)alertWindow
 {
   if (_alertWindow == nil) {
-    _alertWindow = [[UIWindow alloc] initWithWindowScene:RCTKeyWindow().windowScene];
+    UIWindowScene *scene = RCTKeyWindow().windowScene;
+    if (scene != nil) {
+      _alertWindow = [[UIWindow alloc] initWithWindowScene:scene];
+    } else {
+      _alertWindow = [[UIWindow alloc] initWithFrame:UIScreen.mainScreen.bounds];
+    }
 
     if (_alertWindow) {
       _alertWindow.rootViewController = [UIViewController new];