Fix OnPlatform SourceGen for abstract types and protected constructors (dotnet#33214)

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## Description

Fixes two XAML SourceGen issues reported:

1. **"Cannot create an instance of the abstract type or interface
'Brush'"** - When `OnPlatform<Brush>` was used with
`<OnPlatform.Default>` syntax and no matching platform.

2. **"error CS0122: 'View.View()' is inaccessible due to its protection
level"** - When `OnPlatform<View>` was used and no matching platform.

### Example XAML that was failing:

```xml
<OnPlatform x:Key="RadEntryInvalidBorderBrush" x:TypeArguments="Brush">
    <On Platform="WinUI">
        <LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
            <LinearGradientBrush.GradientStops>
                <GradientStop Offset="0.00" Color="{StaticResource RadOnAppSurfaceColorAlpha6}" />
                <GradientStop Offset="0.93" Color="{StaticResource RadOnAppSurfaceColorAlpha6}" />
                <GradientStop Offset="0.93" Color="{StaticResource RadErrorColor}" />
                <GradientStop Offset="1.00" Color="{StaticResource RadErrorColor}" />
            </LinearGradientBrush.GradientStops>
        </LinearGradientBrush>
    </On>
    <OnPlatform.Default>
        <SolidColorBrush Color="{StaticResource RadErrorColor}" />
    </OnPlatform.Default>
</OnPlatform>

<OnPlatform x:TypeArguments="View">
    <On Platform="WinUI">
        <telerikMauiControls:RadBorder Grid.Row="2"
                                        BackgroundColor="{DynamicResource RadAIPromptInputViewFooterBackgroundColor}" />
    </On>
</OnPlatform>
```

## Root Cause

Two issues in `SimplifyOnPlatformVisitor.cs`:

1. `GetDefault()` only checked for `Default` property with empty
namespace (`new XmlName("", "Default")`), but when using
`<OnPlatform.Default>` element syntax, the property is stored with the
MAUI namespace URI.

2. When no matching platform and no Default was found, the code created
an `ElementNode` from the `x:TypeArguments` type, which later failed
when `CreateValuesVisitor` tried to instantiate abstract types or types
with protected constructors.

## Fix

1. Updated `GetDefault()` to check both empty namespace (attribute
syntax) and MAUI namespace (element syntax `<OnPlatform.Default>`)

2. When no matching platform AND no Default is found, skip
simplification and keep the `OnPlatform` element for runtime resolution
instead of trying to create a default value. This is the safe approach
because the runtime `OnPlatform<T>` correctly returns `default(T)`.

## Tests Added

Added 6 new tests in `OnPlatformAbstractTypes.cs` covering:
- `OnPlatformWithAbstractBrushTypeUsesDefault` - Brush with Default on
non-matching platform
- `OnPlatformWithAbstractBrushTypeMatchesPlatform` - Brush when platform
matches
- `OnPlatformWithViewTypeUsesDefault` - View with Default on
non-matching platform
- `OnPlatformWithViewTypeMatchesPlatform` - View when platform matches
- `OnPlatformWithAbstractTypeNoDefaultNoMatchingPlatform` - Brush
without Default on non-matching platform
- `OnPlatformWithProtectedCtorTypeNoDefaultNoMatchingPlatform` - View
without Default on non-matching platform

Author: StephaneDelcroix

src/Controls/src/SourceGen/Visitors/CreateValuesVisitor.cs

@@ -38,6 +38,17 @@ public static void CreateValue(ElementNode node, IndentedTextWriter writer, IDic
 		if (!node.XmlType.TryResolveTypeSymbol(null, compilation, xmlnsCache, Context.TypeCache, out var type) || type is null)
 			return;
 
+		// Handle OnPlatform default value nodes - generate default(T) instead of instantiation
+		// This is used when OnPlatform has no matching platform and no Default specified
+		if (node.IsOnPlatformDefaultValue)
+		{
+			var variableName = NamingHelpers.CreateUniqueVariableName(Context, type);
+			writer.WriteLine($"{type.ToFQDisplayString()} {variableName} = default;");
+			variables[node] = new LocalVariable(type, variableName);
+			node.RegisterSourceInfo(Context, writer);
+			return;
+		}
+
 		//x:Array
 		if (type.Equals(compilation.GetTypeByMetadataName("Microsoft.Maui.Controls.Xaml.ArrayExtension"), SymbolEqualityComparer.Default))
 		{

src/Controls/src/Xaml/SimplifyOnPlatformVisitor.cs

@@ -87,12 +87,17 @@ public void Visit(ElementNode node, INode parentNode)
 			bool hasKey = node.Properties.TryGetValue(XmlName.xKey, out keyNode);
 
 			// If no value found for target platform and no Default,
-			// create a default value node if we have x:TypeArguments
+			// create a default value node if we have x:TypeArguments.
+			// Mark it as IsOnPlatformDefaultValue so SourceGen can generate default(T)
+			// instead of trying to instantiate the type (which fails for abstract types
+			// or types with protected constructors).
 			if (onNode == null && node.XmlType.TypeArguments != null && node.XmlType.TypeArguments.Count > 0)
 			{
-				// Create default value for the type (to match OnPlatform<T> runtime behavior)
 				var typeArg = node.XmlType.TypeArguments[0];
-				var elementNode = new ElementNode(typeArg, typeArg.NamespaceUri, node.NamespaceResolver, node.LineNumber, node.LinePosition);
+				var elementNode = new ElementNode(typeArg, typeArg.NamespaceUri, node.NamespaceResolver, node.LineNumber, node.LinePosition)
+				{
+					IsOnPlatformDefaultValue = true
+				};
 				onNode = elementNode;
 			}
 
@@ -190,8 +195,11 @@ INode GetOnNode(ElementNode onPlatform, string target)
 
 		INode GetDefault(ElementNode onPlatform)
 		{
+			// Check both empty namespace (attribute syntax) and MAUI namespace (element syntax)
 			if (onPlatform.Properties.TryGetValue(new XmlName("", "Default"), out INode defaultNode))
 				return defaultNode;
+			if (onPlatform.Properties.TryGetValue(new XmlName(XamlParser.MauiUri, "Default"), out defaultNode))
+				return defaultNode;
 			return null;
 		}
 

src/Controls/src/Xaml/XamlNode.cs

@@ -110,6 +110,13 @@ class ElementNode(XmlType type, string namespaceURI, IXmlNamespaceResolver names
 	public XmlType XmlType { get; } = type;
 	public string NamespaceURI { get; } = namespaceURI;
 	public NameScopeRef NameScopeRef { get; set; }
+	
+	/// <summary>
+	/// When true, this node represents a default value for an OnPlatform element
+	/// where no matching platform was found. SourceGen should generate default(T)
+	/// instead of trying to instantiate the type.
+	/// </summary>
+	public bool IsOnPlatformDefaultValue { get; set; }
 
 	public override void Accept(IXamlNodeVisitor visitor, INode parentNode)
 	{
@@ -150,7 +157,8 @@ public override INode Clone()
 	{
 		var clone = new ElementNode(XmlType, NamespaceURI, NamespaceResolver, LineNumber, LinePosition)
 		{
-			IgnorablePrefixes = IgnorablePrefixes
+			IgnorablePrefixes = IgnorablePrefixes,
+			IsOnPlatformDefaultValue = IsOnPlatformDefaultValue
 		};
 		foreach (var kvp in Properties)
 			clone.Properties.Add(kvp.Key, kvp.Value.Clone());