[a11y] TreeView: focused row scrolls out of the viewport in virtualized examples

## Problem
On the "Virtualized (Long Lists)" and "Virtualized with Cascade Selection" examples, arrowing through tree items causes the tree to march up out of the visible area. The focused row ends up off screen, so keyboard and screen reader users can't see which item has focus.

## Steps to Reproduce

1. Open https://pr-2475.d15792l1n26ww3.amplifyapp.com/components/TreeView/#virtualized-with-cascade-selection
2. Move focus into the tree.
3. Press Down Arrow repeatedly to move through the items.
4. Watch the position of the tree and the focused row.

## Expected Behavior
The focused tree item stays within the visible viewport as the user arrows through the list.

## Actual Behavior
Each arrow key press scrolls the nearest scrollable ancestor, pushing the tree — and the focused row with it — out of view.