[EuiTreeView] Migrate from class to function component

[weronikaolejniczak]

EuiTreeView renders a hierarchical tree with expandable nodes, keyboard navigation, and ARIA tree semantics.

• File: components/tree_view/tree_view.tsx
• Class size: 254 lines
• State: openItems, activeItem, treeID, expandChildNodes
• Lifecycle: componentDidUpdate

Migration notes:

Recursive rendering pattern with multiple state fields. componentDidUpdate manages expand-all behavior. Wrapped with withEuiTheme.

Acceptance criteria:

• Convert to function component with hooks
• Replace withEuiTheme with useEuiTheme
• Maintain keyboard navigation and ARIA tree semantics
• All existing tests pass