[EuiComboBoxInput] Migrate from class to function component

`EuiComboBoxInput` renders the input field, selected option pills, and handles text input within `EuiComboBox`. Internal sub-component.

- **File:** `components/combo_box/combo_box_input/combo_box_input.tsx`
- **Class size:** 333 lines
- **State:** `inputWidth`, `hasFocus`
- **Lifecycle:** `componentDidUpdate`

**Migration notes:**

Tightly coupled to `EuiComboBox`. Should be migrated together. The `componentDidUpdate` manages focus and input width — maps to `useEffect`.

**Acceptance criteria:**

- [ ] Convert to function component with hooks
- [ ] Migrate as part of the ComboBox family
- [ ] All existing tests pass