Implement coordinator role filtering in ContactListService

Use `Future.wait([repo.getAssignedMembers(id), repo.getSupervisedPeerMentors(id)])` to issue both queries concurrently. After awaiting, merge with a `LinkedHashSet` keyed on `contact.id` to deduplicate while preserving insertion order, then convert back to a sorted list. Sort by `contact.displayName.toLowerCase()` for consistent ordering. For error propagation, let the exception from `Future.wait` bubble naturally — do not catch and rethrow unless you need to add context.

If adding context is desired (e.g. for observability), use `throw ContactListServiceException('Coordinator contact fetch failed', cause: e)` with a typed exception class. Avoid loading both results into a flat list before deduplicating — this wastes memory on large contact sets. The coordinator's own contact record should NOT be included in the result (the list shows contacts they manage, not themselves).

Write unit tests using flutter_test with a FakeContactRepository: (1) both sub-queries return results — assert merged and deduplicated list, (2) a contact appears in both sub-queries — assert it appears exactly once in output, (3) both sub-queries return empty — assert empty list returned, (4) only members sub-query returns results — assert correct list, (5) only peer mentors sub-query returns results — assert correct list, (6) one sub-query throws — assert exception propagates with context, (7) assert Future.wait concurrency by recording call order in the fake. Coverage target: 100% on getContactsForCoordinator. All tests must be deterministic.