feat(VTreeview): add aria-label and aria-expanded to expand/collapse buttons

[mmustafasenoglu]

Description

Adds accessibility attributes to the expand/collapse buttons in VTreeview. Screen readers will now announce "Expand [item]" / "Collapse [item]" instead of just "Button".

Fixes #21585

Motivation

Currently, screen readers announce the expand/collapse buttons in VTreeview simply as "Button" without any context about what the button does. This makes the treeview inaccessible for users relying on assistive technologies.

Changes

• Added isOpen prop to VTreeviewItem to receive the open state from parent
• VTreeviewGroup now passes its isOpen state to VTreeviewItem via defaults
• Added aria-label ("Expand [title]" / "Collapse [title]") to the toggle button
• Added aria-expanded attribute to indicate the current state

Before

<button>▶</button>  <!-- Screen reader: "Button" -->

After

<button aria-label="Expand Red" aria-expanded="false">▶</button>
<!-- Screen reader: "Expand Red, collapsed" -->