Mocks next-themes' useTheme so the component is testable in Node.
Mocks the radix DropdownMenu primitives to render trigger + items
inline instead of through a portal. Coverage:
- Rendered markup picks the correct icon + label for each theme
('light' → Sun, 'dark' → Moon, 'system' / undefined → Monitor).
- All three menu items render under the trigger.
- Each menu item's onClick calls setTheme with the matching value.
Walks the React element tree to grab the onClick handlers without
needing a DOM — keeps the existing react-dom/server testing setup.
Total tests: 92.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
