feat(dfu): add firmware file selection and validation

test/service/firmware_file_selection_service_test.dart

@@ -0,0 +1,132 @@
+import 'dart:typed_data';
+
+import 'package:abawo_bt_app/model/firmware_file_selection.dart';
+import 'package:abawo_bt_app/model/shifter_types.dart';
+import 'package:abawo_bt_app/service/firmware_file_selection_service.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('FirmwareFileSelectionService', () {
+    test('prepares v1 metadata for selected .bin firmware', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.BIN',
+            filePath: '/tmp/firmware.BIN',
+            fileBytes: Uint8List.fromList(<int>[1, 2, 3, 4]),
+          ),
+        ),
+        sessionIdGenerator: () => 0x1AB,
+      );
+
+      final result = await service.selectAndPrepareDfuV1();
+      expect(result.isSuccess, isTrue);
+
+      final firmware = result.firmware!;
+      expect(firmware.fileName, 'firmware.BIN');
+      expect(firmware.filePath, '/tmp/firmware.BIN');
+      expect(firmware.fileBytes, <int>[1, 2, 3, 4]);
+      expect(firmware.metadata.totalLength, 4);
+      expect(firmware.metadata.crc32, 0xB63CFBCD);
+      expect(firmware.metadata.sessionId, 0xAB);
+      expect(firmware.metadata.flags, universalShifterDfuFlagNone);
+    });
+
+    test('returns canceled result when user dismisses picker', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(selection: null),
+      );
+
+      final result = await service.selectAndPrepareDfuV1();
+
+      expect(result.isSuccess, isFalse);
+      expect(result.isCanceled, isTrue);
+      expect(result.failure?.reason, FirmwareSelectionFailureReason.canceled);
+    });
+
+    test('rejects unsupported extension', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.hex',
+            fileBytes: Uint8List.fromList(<int>[1]),
+          ),
+        ),
+      );
+
+      final result = await service.selectAndPrepareDfuV1();
+
+      expect(result.isSuccess, isFalse);
+      expect(result.failure?.reason,
+          FirmwareSelectionFailureReason.unsupportedExtension);
+    });
+
+    test('rejects empty payload', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: Uint8List(0),
+          ),
+        ),
+      );
+
+      final result = await service.selectAndPrepareDfuV1();
+
+      expect(result.isSuccess, isFalse);
+      expect(result.failure?.reason, FirmwareSelectionFailureReason.emptyFile);
+    });
+
+    test('generates session id per run', () async {
+      var nextSession = 9;
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: Uint8List.fromList(<int>[10]),
+          ),
+        ),
+        sessionIdGenerator: () => nextSession++,
+      );
+
+      final first = await service.selectAndPrepareDfuV1();
+      final second = await service.selectAndPrepareDfuV1();
+
+      expect(first.firmware?.metadata.sessionId, 9);
+      expect(second.firmware?.metadata.sessionId, 10);
+    });
+
+    test('maps picker read failure to explicit validation error', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: null,
+          error: const FormatException('broken pick payload'),
+        ),
+      );
+
+      final result = await service.selectAndPrepareDfuV1();
+
+      expect(result.isSuccess, isFalse);
+      expect(result.failure?.reason, FirmwareSelectionFailureReason.readFailed);
+      expect(result.failure?.message, contains('broken pick payload'));
+    });
+  });
+}
+
+class _FakeFirmwareFilePicker implements FirmwareFilePicker {
+  _FakeFirmwareFilePicker({
+    required this.selection,
+    this.error,
+  });
+
+  final FirmwarePickerSelection? selection;
+  final Object? error;
+
+  @override
+  Future<FirmwarePickerSelection?> pickFirmwareFile() async {
+    if (error != null) {
+      throw error!;
+    }
+    return selection;
+  }
+}