test(bridge): inject the lookup cache persist debounce

The lookup cache test waited out the 500 ms production debounce four
times (2.7 s). A @visibleForTesting setter lets the test use a 10 ms
debounce while keeping the same 50 to 250 ms margin after the timer
fires for the background encode. The production default stays 500 ms
and the assertions are unchanged.

lib/services/platform_bridge.dart

@@ -145,7 +145,9 @@ class PlatformBridge {
   static const _bridgeCacheMaxBytes = 8 * 1024 * 1024;
   static const _persistentCacheMaxEntryBytes = 1024 * 1024;
   static const _persistentCacheMaxBytes = 2 * 1024 * 1024;
-  static const _lookupCachePersistDebounce = Duration(milliseconds: 500);
+  static const _defaultLookupCachePersistDebounce = Duration(milliseconds: 500);
+  static Duration _lookupCachePersistDebounce =
+      _defaultLookupCachePersistDebounce;
   static const _metadataPersistentCacheKey = 'bridge_metadata_lookup_cache_v1';
   static const _downloadProgressEvents = EventChannel(
     'com.zarz.spotiflac/download_progress_stream',
@@ -574,6 +576,13 @@ class PlatformBridge {
     }
   }
 
+  /// Shortens the lookup cache persist debounce so tests need not wait the
+  /// production delay. Null restores the default.
+  @visibleForTesting
+  static set lookupCachePersistDebounceForTesting(Duration? value) {
+    _lookupCachePersistDebounce = value ?? _defaultLookupCachePersistDebounce;
+  }
+
   static void _scheduleLookupCachePersist(
     Map<String, _BridgeCacheEntry<Map<String, dynamic>>> cache,
     String prefsKey,

test/platform_bridge_lookup_cache_test.dart

@@ -13,16 +13,22 @@ void main() {
   final messenger =
       TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger;
 
+  // The waits below keep the original margin after the debounce fires (50 to
+  // 250 ms for the background encode) while skipping the 500 ms default.
+  const debounce = Duration(milliseconds: 10);
+
   setUp(() async {
     SharedPreferences.setMockInitialValues({});
     messenger.setMockMethodCallHandler(channel, (_) async => null);
     await PlatformBridge.clearTrackCache();
+    PlatformBridge.lookupCachePersistDebounceForTesting = debounce;
   });
 
   tearDown(() async {
     messenger.setMockMethodCallHandler(channel, (_) async => null);
     await PlatformBridge.clearTrackCache();
     messenger.setMockMethodCallHandler(channel, null);
+    PlatformBridge.lookupCachePersistDebounceForTesting = null;
   });
 
   Map<String, dynamic> collection([int count = 2]) => {
@@ -57,7 +63,7 @@ void main() {
       expect(results.last['text'], isNot('changed'));
       expect((await load())['text'], isNot('changed'));
       expect(calls, 2);
-      await Future<void>.delayed(const Duration(milliseconds: 550));
+      await Future<void>.delayed(debounce + const Duration(milliseconds: 50));
       final prefs = await SharedPreferences.getInstance();
       final stored = prefs.getString('bridge_metadata_lookup_cache_v1');
       expect(stored == null || jsonDecode(stored) is Map, true);
@@ -95,7 +101,7 @@ void main() {
       });
       await PlatformBridge.getProviderMetadata('example', 'album', 'large');
       await PlatformBridge.getProviderMetadata('example', 'track', 'small');
-      await Future<void>.delayed(const Duration(milliseconds: 650));
+      await Future<void>.delayed(debounce + const Duration(milliseconds: 150));
       final prefs = await SharedPreferences.getInstance();
       final stored = prefs.getString('bridge_metadata_lookup_cache_v1')!;
       expect(utf8.encode(stored).length, lessThanOrEqualTo(2 * 1024 * 1024));
@@ -120,7 +126,7 @@ void main() {
       for (final id in ['escaped', 'multibyte', 'small']) {
         await PlatformBridge.getProviderMetadata('example', 'album', id);
       }
-      await Future<void>.delayed(const Duration(milliseconds: 750));
+      await Future<void>.delayed(debounce + const Duration(milliseconds: 250));
       final prefs = await SharedPreferences.getInstance();
       final stored =
           jsonDecode(prefs.getString('bridge_metadata_lookup_cache_v1')!)
@@ -137,7 +143,7 @@ void main() {
     for (var index = 0; index < 3; index++) {
       await PlatformBridge.getProviderMetadata('example', 'album', '$index');
     }
-    await Future<void>.delayed(const Duration(milliseconds: 750));
+    await Future<void>.delayed(debounce + const Duration(milliseconds: 250));
     final prefs = await SharedPreferences.getInstance();
     final raw = prefs.getString('bridge_metadata_lookup_cache_v1')!;
     expect(utf8.encode(raw).length, lessThanOrEqualTo(2 * 1024 * 1024));