[Windows] Allow to remove shadows at runtime (#31452)

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### Description of Change

This PR attempts to address
https://github.com/dotnet/maui/pull/18337/files#r2314339635. That is, it
makes sure that shadow is removed from Border when `Border.Shadow =
null` is assigned.

I took inspiration in Android implementation:


https://github.com/dotnet/maui/blob/718adb7f4f9ad83bcd3ead1627b6fba329837b2a/src/Core/src/Platform/Android/WrapperView.cs#L61-L110

### Test effect

My
`src\Controls\tests\TestCases.WinUI.Tests\snapshots\windows\Issue24414Test_5.png`
changes like this:

| Before | After |
|--------|--------|
| <img width="1010" height="729" alt="image"
src="https://github.com/user-attachments/assets/13e70b02-acab-4ef2-a4d0-f799d60748d2"
/> | <img width="1264" height="952" alt="image"
src="https://github.com/user-attachments/assets/865c4f68-ef05-4d25-a5c6-14c5d96a4872"
/>|

Author: jfversluis

src/Controls/tests/TestCases.WinUI.Tests/snapshots/windows/Issue24414Test_5.png

@@ -167,13 +167,20 @@ void UpdateBorder()
 
 		partial void ShadowChanged()
 		{
-			if (HasShadow)
+			if (Shadow?.Paint is { })
 			{
-				UpdateShadowAsync().FireAndForget(IPlatformApplication.Current?.Services?.CreateLogger(nameof(WrapperView)));
+				if (HasShadow)
+				{
+					UpdateShadowAsync().FireAndForget(IPlatformApplication.Current?.Services?.CreateLogger(nameof(WrapperView)));
+				}
+				else
+				{
+					CreateShadowAsync().FireAndForget(IPlatformApplication.Current?.Services?.CreateLogger(nameof(WrapperView)));
+				}
 			}
 			else
 			{
-				CreateShadowAsync().FireAndForget(IPlatformApplication.Current?.Services?.CreateLogger(nameof(WrapperView)));
+				DisposeShadow();
 			}
 		}