fix(extensions): serialize verification callbacks

test/platform_bridge_session_grant_test.dart

@@ -0,0 +1,54 @@
+import 'dart:async';
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/platform_bridge.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  const backendChannel = MethodChannel('com.zarz.spotiflac/backend');
+
+  tearDown(() {
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(backendChannel, null);
+  });
+
+  test('duplicate session grant completions share one native call', () async {
+    final nativeResult = Completer<bool>();
+    var callCount = 0;
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(backendChannel, (call) async {
+          if (call.method != 'completeExtensionSessionGrant') return null;
+          callCount++;
+          return nativeResult.future;
+        });
+
+    final events = <ExtensionSessionGrantEvent>[];
+    final subscription = PlatformBridge.extensionSessionGrantEvents().listen(
+      events.add,
+    );
+    addTearDown(subscription.cancel);
+
+    final first = PlatformBridge.completeExtensionSessionGrant(
+      ' qobuz-web ',
+      'grant-value',
+    );
+    final second = PlatformBridge.completeExtensionSessionGrant(
+      'QOBUZ-WEB',
+      'grant-value',
+    );
+
+    expect(identical(first, second), isTrue);
+    expect(callCount, 1);
+
+    nativeResult.complete(true);
+    expect(await Future.wait([first, second]), [isTrue, isTrue]);
+    await Future<void>.delayed(Duration.zero);
+
+    expect(callCount, 1);
+    expect(events, hasLength(1));
+    expect(events.single.extensionId, 'qobuz-web');
+    expect(events.single.success, isTrue);
+  });
+}