Add tests for ListBox selection synchronization (item→SelectedIndex and SelectedIndex→IsSelected)

[Copilot]

The OnItemSelected handler (bubbled SelectedEvent → SelectedIndex) and OnSelectionChanged override (container IsSelected sync) had no test coverage.

New tests in ListBoxTests

• ItemIsSelected_ViaIsSelectedProperty_UpdatesListBoxSelectedIndex — sets container.IsSelected = true, asserts the SelectedEvent bubbles to ListBox and updates SelectedIndex
• ItemSelectedEvent_RaisedOnContainer_UpdatesListBoxSelectedIndex — raises SelectedEvent directly on a container, asserts OnItemSelected resolves the correct panel index
• SelectedIndex_Change_Updates_ContainersIsSelected — sets SelectedIndex, asserts exactly that container has IsSelected = true, all others false
• SelectedIndex_Change_Deselects_PreviousContainer — moves selection between indices, asserts old container deselects and new one selects
• SelectedIndex_MinusOne_Deselects_AllContainers — clears selection, asserts all containers deselected

// Example: item-driven path
var listBox = new ListBox();
listBox.Items.Add("A"); listBox.Items.Add("B"); listBox.Items.Add("C");
var container = (ListBoxItem)listBox.ItemsPanelRoot!.Children[1];
container.IsSelected = true;         // raises SelectedEvent → bubbles to ListBox
Assert.Equal(1, listBox.SelectedIndex);

// Example: index-driven path
listBox.SelectedIndex = 2;           // triggers OnSelectionChanged → syncs IsSelected
Assert.True(((ListBoxItem)listBox.ItemsPanelRoot.Children[2]).IsSelected);
Assert.False(((ListBoxItem)listBox.ItemsPanelRoot.Children[0]).IsSelected);

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[Copilot]

Pull request overview

Adds unit test coverage around ListBox selection synchronization (between SelectedIndex on the ListBox and IsSelected / routed selection events on ListBoxItem containers), helping ensure correct selector behavior as the TUI control set evolves.

Changes:

• Add tests verifying ListBoxItem.IsSelected = true (and SelectedEvent) updates ListBox.SelectedIndex.
• Add tests verifying ListBox.SelectedIndex changes update/deselect the corresponding ListBoxItem.IsSelected states, including clearing selection with -1.

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[Copilot]

The comment implies ItemsPanelRoot is created/populated only after items are added. In the current implementation, ListBox applies its Template in the constructor (via Control.Template DP), and ItemsPresenter.OnTemplatedParentChanged() creates ItemsPanelRoot immediately; adding items just repopulates it. Updating this comment (or explicitly calling listBox.ApplyTemplate() in the test) would make the test intent clearer and less coupled to the current initialization order.

[Copilot]

RaiseEvent always overwrites RoutedEventArgs.Source with this, so passing container2 into the RoutedEventArgs(routedEvent, source) constructor here is redundant and can be misleading. Consider using the single-argument RoutedEventArgs(ListBoxItem.SelectedEvent) constructor for clarity.

Suggested change

	container2!.RaiseEvent(new RoutedEventArgs(ListBoxItem.SelectedEvent, container2));
	container2!.RaiseEvent(new RoutedEventArgs(ListBoxItem.SelectedEvent));