TreeListViewItem does not mark KeyEvents as handled

[KennethWhite]

Bug explanation

Description:
In #3333 in the new TreeListViewItem.cs there is an OnKeyDown handler for collapsing/expanding items, however, the handler does not mark the KeyEventArgs as handled.

This leads to unexpected behavior where the event can be handled when this TreeListView is in focus, and then also handled by other event handlers for the arrow keys.

Expected Behavior:
The OnKeyDown handler marks the KeyEventArgs as handled.

https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/blob/2a085a2f0381046a64c074750f9381147d98a4f8/MaterialDesignThemes.Wpf/TreeListViewItem.cs#L177C4-L199C6

Version

5.0.0-ci472