Implement AssignmentHistoryRepository open/closed status and deadline filters

Implement fetchOpenAssignments as a convenience wrapper: return fetchAssignmentsByStatus(mentorId, AssignmentStatus.open) — do not duplicate query logic. For the deadline ordering, use Dart's List.sort with a comparator that handles null due_dates by placing them after non-null ones. Inject a Clock or DateTime provider into the repository (or the mapper) so unit tests can fix 'now' to a known value — avoids flaky tests that depend on wall time. The AssignedContact model should already include all fields from task-008; this task only adds the filter-by-status query methods.

If the Supabase table uses a string column for status (e.g., 'open', 'closed'), map to/from AssignmentStatus enum in the fromJson factory, not in the repository method.

Unit tests with flutter_test and mockito. Test cases: (1) fetchOpenAssignments returns only open-status records; (2) fetchAssignmentsByStatus with open status matches fetchOpenAssignments results; (3) fetchAssignmentsByStatus with closed status returns only closed records; (4) overdue_flag is true when due_date is in the past; (5) overdue_flag is false when due_date is in the future; (6) days_remaining is null when due_date is null; (7) days_remaining is negative for overdue assignments; (8) result ordering puts most imminent deadlines first, null deadlines last; (9) empty result returns empty list. Use DateTime.now() injection or a clock abstraction to make date-dependent assertions deterministic in tests.