Fix MessageBox dialogs not adapting to dark mode in WPF Gallery

[amarinov-msft]

Summary
Fixes an accessibility issue in the WPF Gallery Message Box page. Dialogs shown via System.Windows.MessageBox.Show(...) render as the native Win32 message box, which does not honor the app's Fluent ThemeMode. In Dark mode the dialog stays light, so its text renders below the 4.5:1 contrast minimum required by MAS 4.3.1 and is flagged by Accessibility Insights. This is a platform limitation: MessageBox.ShowCore calls the user32 MessageBox, and the Fluent theme has no template for it, so the dialog cannot adapt to the app theme.

Change
Sample Applications/WPFGallery/Controls/ThemedMessageBox.xaml  /  .xaml.cs
Add a Fluent-themed ThemedMessageBox (a WPF Window) that inherits the application theme, so it adapts to Light/Dark/High Contrast automatically. It reuses the platform's own message-box brushes (MessageBoxBackground, MessageBoxForeground, MessageBoxSeparatorBorderBrush, MessageBoxTopOverlay) and SymbolThemeFontFamily for the severity icon.

Sample Applications/WPFGallery/Views/System/MessageBoxPage.xaml.cs 
Route the page's 7 dialog call sites through ThemedMessageBox instead of System.Windows.MessageBox.

Result
In Dark mode the message-box dialogs now render with the app's dark background and full-contrast Fluent foreground. Light and High Contrast appearance follow the app theme as well. Button semantics and returned MessageBoxResult values are preserved.

Testing

• Built WPFGallery.csproj (0 errors).
• Verified the Message Box page dialogs under Dark mode and confirmed buttons, default/cancel behavior, and return values match the previous dialogs.
• Verified changing through Dark/Light modes

Microsoft Reviewers: Open in CodeFlow