Write unit and widget tests for receipt attachment flow

Structure test files mirroring source structure: receipt_attachment_service_test.dart, receipt_image_compressor_test.dart, receipt_threshold_validator_test.dart, receipt_camera_sheet_test.dart, receipt_attachment_indicator_test.dart. Create a test_helpers/ directory with: a factory for generating test XFile instances (solid-color PNG bytes, no EXIF), and a MockReceiptAttachmentService class using mocktail's Mock. For ReceiptImageCompressor tests, generate a synthetic oversized image in memory using dart:ui or the image package — do not rely on asset files. For ReceiptAttachmentIndicator widget tests, provide a StreamController and manually add events, then call tester.pump() to trigger rebuilds.

Use group() nesting: outer group for class name, inner group for method/scenario name. Avoid test interdependency — each test must be fully independent with its own setup. Add a README comment at the top of each test file explaining what is being tested and what is mocked.

Unit tests (pure Dart, no widget pump needed) for ReceiptAttachmentService, ReceiptImageCompressor, and ReceiptThresholdValidator. Widget tests (WidgetTester) for ReceiptCameraSheet and ReceiptAttachmentIndicator. Use StreamController in tests to emit controlled states from ReceiptAttachmentService mock. Use fake_async for timer-dependent behaviour if any timeouts exist in ReceiptAttachmentService.

Verify state transitions by collecting emitted events from the service stream using expectLater with emitsInOrder. For widget tests, use pump() and pumpAndSettle() carefully — do not use pumpAndSettle() when streams are active as it may time out.