feat(dfu): add connection and MTU preflight checks

test/service/dfu_preflight_test.dart

@@ -0,0 +1,137 @@
+import 'package:abawo_bt_app/controller/bluetooth.dart';
+import 'package:abawo_bt_app/model/shifter_types.dart';
+import 'package:abawo_bt_app/service/shifter_service.dart';
+import 'package:anyhow/anyhow.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('ShifterService.runDfuPreflight', () {
+    test('fails when no active button connection exists', () async {
+      final adapter = _FakeDfuPreflightBluetoothAdapter(
+        currentConnectionState: (ConnectionStatus.disconnected, null),
+        mtuResult: Ok(128),
+      );
+      final service = ShifterService(
+        buttonDeviceId: 'target-device',
+        dfuPreflightBluetooth: adapter,
+      );
+
+      final result = await service.runDfuPreflight();
+
+      expect(result.isOk(), isTrue);
+      final preflight = result.unwrap();
+      expect(preflight.canStart, isFalse);
+      expect(preflight.failureReason,
+          DfuPreflightFailureReason.deviceNotConnected);
+      expect(adapter.requestMtuCallCount, 0);
+    });
+
+    test('fails when connected to a different button', () async {
+      final adapter = _FakeDfuPreflightBluetoothAdapter(
+        currentConnectionState: (ConnectionStatus.connected, 'wrong-device'),
+        mtuResult: Ok(128),
+      );
+      final service = ShifterService(
+        buttonDeviceId: 'target-device',
+        dfuPreflightBluetooth: adapter,
+      );
+
+      final result = await service.runDfuPreflight();
+
+      expect(result.isOk(), isTrue);
+      final preflight = result.unwrap();
+      expect(preflight.canStart, isFalse);
+      expect(preflight.failureReason,
+          DfuPreflightFailureReason.wrongConnectedDevice);
+      expect(adapter.requestMtuCallCount, 0);
+    });
+
+    test('fails when MTU negotiation fails', () async {
+      final adapter = _FakeDfuPreflightBluetoothAdapter(
+        currentConnectionState: (ConnectionStatus.connected, 'target-device'),
+        mtuResult: bail('adapter rejected mtu request'),
+      );
+      final service = ShifterService(
+        buttonDeviceId: 'target-device',
+        dfuPreflightBluetooth: adapter,
+      );
+
+      final result = await service.runDfuPreflight(requestedMtu: 247);
+
+      expect(result.isOk(), isTrue);
+      final preflight = result.unwrap();
+      expect(preflight.canStart, isFalse);
+      expect(
+          preflight.failureReason, DfuPreflightFailureReason.mtuRequestFailed);
+      expect(preflight.message, contains('adapter rejected mtu request'));
+      expect(adapter.requestedMtuValues, [247]);
+    });
+
+    test('fails when negotiated MTU is too low for 64-byte frame writes',
+        () async {
+      final adapter = _FakeDfuPreflightBluetoothAdapter(
+        currentConnectionState: (ConnectionStatus.connected, 'target-device'),
+        mtuResult: Ok(universalShifterDfuMinimumMtu - 1),
+      );
+      final service = ShifterService(
+        buttonDeviceId: 'target-device',
+        dfuPreflightBluetooth: adapter,
+      );
+
+      final result = await service.runDfuPreflight();
+
+      expect(result.isOk(), isTrue);
+      final preflight = result.unwrap();
+      expect(preflight.canStart, isFalse);
+      expect(preflight.failureReason, DfuPreflightFailureReason.mtuTooLow);
+      expect(preflight.negotiatedMtu, universalShifterDfuMinimumMtu - 1);
+      expect(preflight.requiredMtu, universalShifterDfuMinimumMtu);
+    });
+
+    test('passes when connected to target and MTU is sufficient', () async {
+      final adapter = _FakeDfuPreflightBluetoothAdapter(
+        currentConnectionState: (ConnectionStatus.connected, 'target-device'),
+        mtuResult: Ok(128),
+      );
+      final service = ShifterService(
+        buttonDeviceId: 'target-device',
+        dfuPreflightBluetooth: adapter,
+      );
+
+      final result = await service.runDfuPreflight();
+
+      expect(result.isOk(), isTrue);
+      final preflight = result.unwrap();
+      expect(preflight.canStart, isTrue);
+      expect(preflight.failureReason, isNull);
+      expect(preflight.negotiatedMtu, 128);
+      expect(preflight.requestedMtu, universalShifterDfuPreferredMtu);
+    });
+  });
+}
+
+class _FakeDfuPreflightBluetoothAdapter
+    implements DfuPreflightBluetoothAdapter {
+  _FakeDfuPreflightBluetoothAdapter({
+    required this.currentConnectionState,
+    required Result<int> mtuResult,
+  }) : _mtuResult = mtuResult;
+
+  @override
+  final (ConnectionStatus, String?) currentConnectionState;
+
+  final Result<int> _mtuResult;
+
+  int requestMtuCallCount = 0;
+  final List<int> requestedMtuValues = <int>[];
+
+  @override
+  Future<Result<int>> requestMtuAndGetValue(
+    String deviceId, {
+    required int mtu,
+  }) async {
+    requestMtuCallCount += 1;
+    requestedMtuValues.add(mtu);
+    return _mtuResult;
+  }
+}