[iOS/macCatalyst] [Candidate Fix] Editor shadow and theme regression caused by BackgroundColor reset on initial handler connection (#35343)

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### Root Cause of the Regression
PR #34741's MapBackground overrides call platformView.BackgroundColor =
null during initial handler connection (when Background is null by
default), making Editor controls transparent at startup — breaking
shadow rendering and app-theme color bindings.


### Description of Change
In EditorHandler.iOS.cs, replaced platformView.BackgroundColor = null
with platformView.BackgroundColor = ColorExtensions.BackgroundColor
(UIColor.SystemBackground) so the Editor restores the system's adaptive
default color instead of becoming transparent. This preserves shadow
rendering on initial load and correctly responds to light/dark theme
changes.

 
### Issues Fixed
UI Test

iOS:
VerifyEditorPlaceholderWithShadow, VerifyEditor_WithShadow,
ShadowsDontRespectControlShape
 
Mac:
VerifyEditorPlaceholderWithShadow, VerifyEditor_WithShadow,
ShadowsDontRespectControlShape,
EditorAndEntryInputFieldsShouldChangeColorsOnAppThemeChange,
EntryAndEditorPlaceholderTextColorAppThemeBindingUpdatesOnThemeChange,
EntryAndEditorTextColorAppThemeBindingUpdatesOnThemeChange - Fails due
to this PR - #34741

Author: Shalini-Ashokan

src/Core/src/Handlers/Editor/EditorHandler.iOS.cs

@@ -3,6 +3,7 @@
 using Foundation;
 using Microsoft.Maui.Devices;
 using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Platform;
 using UIKit;
 
 namespace Microsoft.Maui.Handlers
@@ -98,7 +99,7 @@ public static void MapBackground(IEditorHandler handler, IEditor editor)
 			else if (editor.Background.IsNullOrEmpty())
 			{
 				platformView.RemoveBackgroundLayer();
-				platformView.BackgroundColor = null;
+				platformView.BackgroundColor = ColorExtensions.BackgroundColor;
 			}
 			else
 			{