Build MentorMultiSelectWidget with search and chip UI

Use a ConsumerStatefulWidget to watch the mentor list from a Riverpod provider, filtered by the coordinator's chapterId. Store selected mentor IDs in a local Set _selectedIds in the widget state — Set gives O(1) lookup for the checkbox state. Implement debounce using a Timer: cancel the previous timer on each keystroke and start a new 300ms timer. For the chip row, wrap Chips in a Wrap widget inside a ConstrainedBox with maxHeight to avoid unbounded growth pushing the list off screen.

The select-all button should operate on the current _filteredList (post-search) so it is context-aware. Ensure the widget is fully self-contained with no hardcoded chapter ID — receive it via constructor parameter. Register the mentor data provider in Riverpod so tests can override it with a mock list without touching real Supabase. Keep the widget reusable: it should work in any flow that needs multi-mentor selection, not just the bulk proxy flow.

Widget tests (task-011) must cover: (1) initial render shows full mentor list from mock data, (2) typing in search input filters list after 300ms debounce, (3) tapping a mentor item adds a chip and checks the item, (4) tapping chip delete removes selection and unchecks item, (5) select-all selects all visible items up to 50, (6) selecting 51st item is blocked, (7) empty state shows when no match found, (8) onSelectionChanged emits correct list on each change, (9) semantics tree contains correct labels. Use flutter_test with fake async for debounce timing tests and mock the mentor data source via a Riverpod override.