[EuiTabbedContent] Migrate from class to function component

`EuiTabbedContent` manages tab selection state and renders tab navigation with content panels. A commonly used container component.

- **File:** `components/tabs/tabbed_content/tabbed_content.tsx`
- **Class size:** 135 lines
- **State:** `selectedTabId`, `inFocus`
- **Lifecycle:** None

**Migration notes:**

Straightforward — state fields become `useState`, no lifecycle methods to convert. Constructor initialization maps to `useState` initial values.

**Acceptance criteria:**

- [ ] Convert to function component with `useState`
- [ ] Maintain controlled/uncontrolled tab selection behavior
- [ ] All existing tests pass