[EuiDelayHide] Migrate from class to function component

[weronikaolejniczak]

EuiDelayHide delays hiding its children for a specified duration after the hide prop becomes true. Used for loading indicator UX.

• File: components/delay_hide/delay_hide.tsx
• Class size: 76 lines
• State: hide, countdownExpired
• Lifecycle: getDerivedStateFromProps, componentDidMount, componentDidUpdate, componentWillUnmount

Migration notes:

Small but has the full lifecycle suite including getDerivedStateFromProps. Timer management maps to useEffect cleanup. The derived state pattern needs careful conversion.

Acceptance criteria:

• Convert to function component with hooks
• Maintain delay timing behavior
• All existing tests pass