fix(controls): Fixes focus issue in MessageBox and adds option to specify focus target.

[Koichi-Kobayashi]

Pull request type

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

When MessageBox is displayed, buttons do not receive focus, making keyboard navigation impossible without first clicking a button with the mouse.
(Ver 4.1.0)

Issue Number: N/A

What is the new behavior?

1. Automatically sets focus to the first available button when MessageBox is displayed
2. Adds DefaultFocusedButton property to allow developers to specify which button should receive focus

• Added SetFocusToFirstAvailableButton() method to traverse visual tree and set focus
• Added DefaultFocusedButton dependency property (nullable MessageBoxButton)
• Modified OnLoaded() to set focus after visual tree is fully constructed
• Falls back to automatic selection if specified button is unavailable

Other information

Example

// Automatic focus (default)
var msg = new MessageBox { Title = "Title", Content = "Content" };
await msg.ShowDialogAsync();

// Specify focus target
var msg = new MessageBox 
{ 
    Title = "Title", 
    Content = "Content",
    DefaultFocusedButton = MessageBoxButton.Secondary 
};
await msg.ShowDialogAsync();