Define Bufdir API client interface contract

Use Dart's sealed class pattern for SubmitReportResponse to get exhaustive pattern matching: sealed class SubmitReportResponse {} / class SubmitReportSuccess extends SubmitReportResponse { final BufdirSubmissionResult result; } / class SubmitReportFailure extends SubmitReportResponse { final BufdirSubmissionError error; }. This eliminates null checks in the orchestration layer. Define BufdirSubmissionError as an enum or sealed class covering the distinct failure modes. The interface design should anticipate that the Bufdir API negotiation is not complete — keep SubmitReportRequest flexible (Map for payload) so the schema can be updated without changing the interface.

Consider adding a validate() method to SubmitReportRequest that checks required fields before submission to provide early validation. Place all types in lib/features/bufdir/data/models/bufdir_submission_models.dart and the interface in lib/features/bufdir/domain/repositories/i_export_submission_client.dart following clean architecture conventions.

Unit tests for the model classes only (no integration tests at this stage): (1) BufdirSubmissionResult serialization — assert toJson/fromJson round-trip preserves all fields including null referenceNumber; (2) SubmitReportRequest serialization round-trip; (3) BufdirSubmissionStatus enum coverage — assert all enum values are handled in a switch expression (exhaustiveness check via Dart sealed class or explicit default throw); (4) compile-time test: a mock implementation of IExportSubmissionClient compiles without error. No network calls in any tests for this task.