Address review feedback: add Superview null guard and enable MacCatalyst test

- Add null-conditional on Superview in InvalidateMeasure to prevent
  NullReferenceException during view lifecycle transitions
- Enable header/footer resize test for MacCatalyst since the fix
  applies to both iOS and MacCatalyst

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

Author: kubaflo

src/Controls/src/Core/Compatibility/Handlers/Shell/iOS/ShellFlyoutHeaderContainer.cs

@@ -39,7 +39,8 @@ void IPlatformMeasureInvalidationController.InvalidateAncestorsMeasuresWhenMoved
 
 		bool IPlatformMeasureInvalidationController.InvalidateMeasure(bool isPropagating)
 		{
-			var size = SizeThatFits(new CGSize(Superview.Frame.Width, double.PositiveInfinity));
+			var width = Superview?.Frame.Width ?? Frame.Width;
+			var size = SizeThatFits(new CGSize(width, double.PositiveInfinity));
 			Frame = new CGRect(Frame.X, Frame.Y, size.Width, size.Height);
 			return false;
 		}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/XFIssue/HeaderFooterShellFlyout.cs

@@ -67,7 +67,7 @@ public void AFlyoutTests()
 
 		// verify header and footer react to size changes
 		// On Windows, the stack layout's AutomationId isn't behaving as expected, so the Y position of the first flyout item is used to verify header and footer sizes.
-#if ANDROID || IOS
+#if ANDROID || IOS || MACCATALYST
 
 		App.Tap(ResizeHeaderFooter);
 		var headerSizeSmall = App.WaitForElement("HeaderView").GetRect();