Description

This PR adds a new OnCreateContainer() virtual method to the ViewHandler base class, allowing developers to provide custom WrapperView implementations for container scenarios. This addresses the need for custom screen reader support and other accessibility scenarios where the default WrapperView is insufficient.

Problem

Previously, developers who needed custom container views had to use reflection to access private setters:

// Before: Required reflection to set custom containers
ContainerView = customWrapperView; // ❌ private setter
((WrapperView)ContainerView).Child = PlatformView; // ❌ internal setter

This was particularly problematic for Windows accessibility scenarios where developers needed to create custom automation peers for controls like Border (which is sealed and cannot be inherited).

Solution

The new API provides a clean, supported way to customize container views:

public class MyCustomViewHandler : ViewHandler<MyView, MyPlatformView>
{
    public override WrapperView OnCreateContainer()
    {
        return new CustomWrapperView(); // ✅ Clean API
    }
}

Changes Made

1. Added OnCreateContainer() virtual method to ViewHandler base class that returns WrapperView?
2. Updated ContainerView property from private protected set to protected set for derived class access
3. Updated all platform implementations (Windows, iOS, Android, Tizen) to call OnCreateContainer() before falling back to default WrapperView
4. Added comprehensive unit tests covering both custom container and fallback scenarios
5. Maintained full backward compatibility - existing code continues to work unchanged

Platform Implementation Details

• Windows: Updated ViewHandlerOfT.Windows.cs to use OnCreateContainer() ?? new WrapperView()
• iOS: Updated ViewHandlerOfT.iOS.cs to use OnCreateContainer() ?? new WrapperView(bounds)
• Android: Updated ViewHandlerOfT.Android.cs to use OnCreateContainer() ?? new WrapperView(context)
• Tizen: Updated ViewHandlerOfT.Tizen.cs to use custom container with fallback

Usage Examples

Basic Custom Container

public override WrapperView OnCreateContainer()
{
    return new AccessibleWrapperView 
    { 
        AccessibilityRole = "CustomControl" 
    };
}

Conditional Custom Container

public override WrapperView OnCreateContainer()
{
    if (VirtualView?.AutomationId?.StartsWith("special_") == true)
    {
        return new CustomWrapperView();
    }
    
    return null; // Use default WrapperView
}

Testing

• ✅ All existing unit tests pass (11/11 in AbstractViewHandlerTests)
• ✅ New unit tests verify custom container functionality
• ✅ New unit tests verify fallback to default behavior
• ✅ Cross-platform builds successful (Android, .NET Standard)
• ✅ API compatibility maintained

Fixes #30750.

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