Fixes #4851 - Add DropDownList<TEnum> for type-safe enum selection

[Copilot]

• Fixes Add DropdownList<T> #4851

DropDownList was cumbersome to use with enum sources, requiring manual population and string-to-enum conversion at every call site. DropDownList<TEnum> closes the gap with OptionSelector<TEnum> — the two are now interchangeable for enum-backed selection, differing only in appearance and size.

Proposed Changes/Todos

• Add Terminal.Gui/Views/DropDownListTEnum.cs — new sealed class DropDownList<TEnum> : DropDownList, IValue where TEnum : struct, Enum
  • Constructor auto-populates Source from Enum.GetValues<TEnum>()
  • Typed TEnum? Value property (get parses Text via Enum.TryParse; set writes value.ToString())
  • Typed event EventHandler<EventArgs<TEnum?>>? ValueChanged — mirrors OptionSelector<TEnum> exactly
  • Implements IValue.GetValue() for data binding
• Update DropDownListExample.cs — added DropDownList<DayOfWeekShort> demo in the UICatalog scenario
• Update DropDownListTests.cs — added DropDownListGenericTests (11 tests: source population, value round-trip, event firing, interchangeability assertion)
• Fix CI build error CS0266: IListDataSource.ToList() returns non-generic System.Collections.IList; added explicit .Cast<object?>() in test assertion

Before / After

// Before — manual wiring, string conversion at every call site
var dd = new DropDownList {
    Source = new ListWrapper<string>(new ObservableCollection<string>(Enum.GetNames<DayOfWeek>()))
};
dd.ValueChanged += (_, e) => Process(Enum.Parse<DayOfWeek>(e.NewValue!));

// After — one line, fully type-safe
var dd = new DropDownList<DayOfWeek> { Value = DayOfWeek.Monday };
dd.ValueChanged += (_, e) => Process(e.Value);  // e.Value is DayOfWeek?

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add DropdownList</issue_title>
<issue_description>DropdownList is working great.

But for many scenarios where the list source is from an enum, it's harder to use than it could be.

We should add a DropDownList that works exactly like OptionSelector.

They should be interchangeable except for how they look and their size.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add DropdownList<T> #4851

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[tig]

@copilot ci/cs
Tests are failing

[Copilot]

@copilot ci/cs
Tests are failing

Fixed in 2138108. The build error was CS0266: IListDataSource.ToList() returns System.Collections.IList (non-generic), which can't be implicitly assigned to IList<object?>. Added explicit .Cast<object?>() to the assertion. All 15,129 tests pass.