[EuiFilterSelectItem] Migrate from class to function component

[weronikaolejniczak]

EuiFilterSelectItem renders individual selectable items within EuiFilterGroup. Used in dashboard filter controls.

• File: components/filter_group/filter_select_item.tsx
• Class size: 152 lines
• State: hasFocus
• Lifecycle: componentDidUpdate

Migration notes:

Focus management in componentDidUpdate maps to useEffect. Wrapped with withEuiTheme — can switch to useEuiTheme.

Acceptance criteria:

• Convert to function component with hooks
• Replace withEuiTheme with useEuiTheme
• Maintain focus behavior
• All existing tests pass