Peer Mentor Single Selector search and single-select enforcement

Manage search and selection state in a single Riverpod StateNotifier (SingleSelectorNotifier) so both pieces of state are co-located and can be tested in isolation. Debounce by holding a Timer in the notifier and cancelling it on each new search input event — avoid using StreamTransformer unless it is already established in the project. The filter logic should derive the visible list via a Provider.select() or a derived AutoDisposeProvider watching both the full list and the search query; this avoids re-rendering the full list widget when only the query changes. For the Continue button, pass an onPressed: null value when no mentor is selected — Flutter's ElevatedButton/TextButton automatically renders in a disabled style and is correctly announced as disabled by screen readers.

Store hasAttemptedContinue as a boolean flag in the notifier to control inline error visibility, flipping it true on a Continue tap and false when a valid selection exists.

Write widget tests for: (1) search filtering — type 'anna', assert only rows containing 'anna' (case-insensitive) are visible; (2) debounce — use fake async to verify list does not re-render before 300 ms elapses; (3) selection — tap a row, assert it is visually selected and others are not; (4) single-select enforcement — tap row A, then row B, assert row A is deselected and row B is selected; (5) Continue disabled — assert button is disabled with no selection; (6) Continue enabled — select a mentor, assert button is enabled; (7) validation error — call continue handler with no selection, assert error message is visible; (8) error clears — assert error disappears after selection. Write unit tests for the debounce and filter logic in isolation.