Add retransmission handling

test/providers/helpers/message_delivery_tracker_test.dart

@@ -0,0 +1,37 @@
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_sar_app/providers/helpers/message_delivery_tracker.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  group('MessageDeliveryTracker', () {
+    test('matches pending direct messages by contact', () {
+      final tracker = MessageDeliveryTracker();
+      final alice = Uint8List.fromList(List<int>.filled(32, 0xAA));
+      final bob = Uint8List.fromList(List<int>.filled(32, 0xBB));
+
+      tracker.trackPendingDirectMessage('alice-1', alice);
+      tracker.trackPendingDirectMessage('bob-1', bob);
+      tracker.trackPendingDirectMessage('alice-2', alice);
+
+      expect(tracker.popPendingDirectMessageId(alice), 'alice-1');
+      expect(tracker.popPendingDirectMessageId(bob), 'bob-1');
+      expect(tracker.popPendingDirectMessageId(alice), 'alice-2');
+    });
+
+    test('removeByMessageId clears pending queue state', () {
+      final tracker = MessageDeliveryTracker();
+      final alice = Uint8List.fromList(List<int>.filled(32, 0xAA));
+
+      tracker.trackPendingDirectMessage('alice-1', alice);
+      tracker.mapAckTagToMessageId(42, 'alice-1');
+
+      tracker.removeByMessageId('alice-1');
+
+      expect(tracker.getMessageIdForAck(42), isNull);
+      expect(tracker.popPendingDirectMessageId(alice), isNull);
+    });
+  });
+}