Unit tests for ProxyRegistrationService orchestration paths

Create mock classes at the top of the test file using mocktail's `class MockCoordinatorRoleGuard extends Mock implements CoordinatorRoleGuard {}` pattern — one mock class per collaborator. In setUp(), instantiate a fresh ProxyRegistrationService with injected mocks before each test. Define shared fixture data (valid coordinator ID, valid mentor ID, sample activity template) as top-level constants to keep tests readable. For the duplicate path, ensure the mock returns a DuplicateConflict value object rather than throwing — the service should handle conflicts as control flow, not exceptions.

For the repository error path, use `thenThrow(PostgrestException(message: 'conflict', code: '23505'))` to simulate a unique constraint violation. Document each test's purpose with a comment referencing the acceptance criterion it validates.

This task IS the test implementation. Use flutter_test as the test runner and mocktail for mocking (preferred over mockito due to null-safety ergonomics in Dart 3). Structure tests in a single file: `test/orchestration/proxy_registration_service_test.dart`. Group tests by scenario using `group()`.

Each `test()` block should follow Arrange-Act-Assert structure. Use `when()` and `thenReturn()` / `thenThrow()` from mocktail. Assert return types with `isA()` and `isA()`. Assert mock interaction counts with `verify(() => mock.method()).called(1)` and `verifyNever(() => mock.method())`.