Port .NET MAUI template changes to DeveloperBalance samples

[Copilot]

This PR ports the changes from dotnet/maui#31564 to the DeveloperBalance sample applications in both the 9.0 and 10.0 folders.

Background

The original PR addressed an issue where Runtime XAML and XamlC inflators ignore required fields and properties in DataTemplateSelector classes. While XSG (XAML Source Generator) can handle these in some cases, it fails for complex or long references. The solution replaces compile-time checks (which are ignored) with runtime validation.

Changes Made

Updated ChipDataTemplateSelector.cs in both sample versions:

Before:

public class ChipDataTemplateSelector : DataTemplateSelector
{
    public required DataTemplate SelectedTagTemplate { get; set; }
    public required DataTemplate NormalTagTemplate { get; set; }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        return (item as Tag)?.IsSelected ?? false ? SelectedTagTemplate : NormalTagTemplate;
    }
}

After:

public class ChipDataTemplateSelector : DataTemplateSelector
{
    public DataTemplate? SelectedTagTemplate { get; set; }
    public DataTemplate? NormalTagTemplate { get; set; }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        return ((item as Tag)?.IsSelected ?? false ? SelectedTagTemplate : NormalTagTemplate) 
            ?? throw new InvalidOperationException("DataTemplates SelectedTagTemplate and NormalTagTemplate must be set to a non-null value.");
    }
}

Files Modified

• 9.0/Apps/DeveloperBalance/Pages/Controls/ChipDataTemplateSelector.cs
• 10.0/Apps/DeveloperBalance/Pages/Controls/ChipDataTemplateSelector.cs

The changes ensure compatibility with XAML runtime inflators by replacing the required keyword with nullable types and adding proper runtime validation.

Fixes #673.

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