Fix ListBoxItem visual state stuck when parent ListBox gains/loses focus

[Copilot]

ListBoxItem.UpdateVisualState() reads parentListBox.IsFocused to pick focused vs. unfocused selection colors, but was only invoked on IsSelected/IsFocused changes on the item itself. When the ListBox gained or lost focus (e.g., Tab navigation) without any item property changing, colors would get stuck in the wrong state.

Changes

• ListBox — Override OnGotFocus()/OnLostFocus() to call a new NotifyContainersVisualStateChanged() helper that iterates ItemsPanelRoot.Children and refreshes each ListBoxItem's visual state immediately when the list box focus changes.
• ListBoxItem — Widen UpdateVisualState() from private to internal so ListBox can invoke it.

// ListBox.cs
public override void OnGotFocus()
{
    base.OnGotFocus();
    NotifyContainersVisualStateChanged();  // items now see IsFocused=true
}

public override void OnLostFocus()
{
    base.OnLostFocus();
    NotifyContainersVisualStateChanged();  // items now see IsFocused=false
}

private void NotifyContainersVisualStateChanged()
{
    if (ItemsPanelRoot != null)
    {
        for (int i = 0; i < ItemsPanelRoot.Children.Count; i++)
        {
            if (ItemsPanelRoot.Children[i] is ListBoxItem lbi)
                lbi.UpdateVisualState();
        }
    }
}

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