Fix ListBox coercing UIElement items to string when no ItemTemplate is set

[Copilot]

ListBox.IsItemItsOwnContainerOverride only returns true for ListBoxItem, so arbitrary UIElement items (e.g. Button) are wrapped in a new ListBoxItem. Previously, PrepareContainerForItemOverride always called GetItemText(item) when ItemTemplate is null, setting the container's Content to a string and discarding the element entirely.

Changes

• ListBox.PrepareContainerForItemOverride: adds an else if (item is UIElement) branch that assigns the element directly as Content, bypassing GetItemText. Non-UIElement items continue to fall back to GetItemText.

if (ItemTemplate != null)
{
    lbi.ContentTemplate = ItemTemplate;
    lbi.Content = item;
}
else if (item is UIElement uiElement)
{
    // Preserve UIElement items as content so they render and interact correctly
    lbi.Content = uiElement;
}
else
{
    lbi.Content = GetItemText(item);
}

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[Copilot]

Pull request overview

Fixes ListBox item hosting so that UIElement items (e.g., Button, TextBlock) are not coerced into strings when ItemTemplate is unset, preserving their rendering and interaction behavior within the ListBoxItem container model used for selection.

Changes:

• Update ListBox.PrepareContainerForItemOverride to detect UIElement items and assign them directly to ListBoxItem.Content when ItemTemplate is null.
• Keep existing behavior for non-UIElement items by continuing to fall back to GetItemText(item).

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[Copilot]

Add unit coverage for the new UIElement-item branch (e.g., a TextBlock or Button in Items with ItemTemplate == null). Without a test, it’s easy to regress back to coercing UIElement items via GetItemText, since the existing ListBox render tests only cover string items and templated items.