Make tree view expansion a marker component

[jbuehler23]

Objective

Follow-up to #25821 addressing viridia's review. Expansion was a bool on the row, so nothing could feed it to the accessibility tree. As a marker component, like checked state, a hook can keep the accesskit expanded property in sync.

Part of #23013.

Solution

Added expandable and expanded markers to the ui crate's interaction states, next to the checked ones, with matching hooks that set or clear the accessibility property. The tree view reads the marker instead of a field and the self update observer inserts or removes it. Rows with children are marked expandable by the widget's post update pass, so leaf rows never report a collapsed state.

The expand request event keeps its bool payload since it is a request, not state.

AI disclosure

AI assistance was used to help port these changes and to plan this upstream series from our existing Jackdaw functionality. All changes were reviewed by a human before submission.