Unit tests for AssignmentHistoryRepository and cross-repository integration

The integration test is not a true end-to-end test — it is a structural test confirming the repositories can coexist and share a client without stateful collisions (e.g., no global variables, no singleton misuse). The primary risk this test guards against is a developer accidentally using a module-level Supabase singleton instead of injected dependencies. For the overdue flag determinism, create a simple ClockProvider interface with a realNow implementation and a testNow(DateTime fixed) implementation, and inject it into the mapper. The integration test's Future.wait() pattern mirrors how the BLoC or Riverpod provider will actually call these repositories in production — so if the test passes, the production parallel-fetch pattern is validated by design.

Unit tests in assignment_history_repository_test.dart using flutter_test and mockito/mocktail. Inject a fixed Clock/DateTime for overdue flag tests so results are deterministic regardless of when CI runs. For the integration test in data_layer_integration_test.dart: create a single shared MockSupabaseClient, instantiate all four repositories with it, define separate when() stubs for each repository's query (using argument matchers that distinguish between table names), call all four via Future.wait(), and assert each resolved to its expected type. Verify that each repository's stub was called exactly once and no other repository's stub was called more than expected.

Group tests using Dart's group() for readability.