test: add comprehensive Go backend and Dart model test suites

- Add 16 Go supplement test files covering extension runtime, providers,
  health checks, lyrics, metadata, HTTP utils, library scan, and more
- Add Dart model/utils test suite (test/models_and_utils_test.dart)
- Update settings.g.dart with deduplicateDownloads serialization

lib/models/settings.g.dart

@@ -78,6 +78,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       json['lyricsMultiPersonWordByWord'] as bool? ?? false,
   musixmatchLanguage: json['musixmatchLanguage'] as String? ?? '',
   lastSeenVersion: json['lastSeenVersion'] as String? ?? '',
+  deduplicateDownloads: json['deduplicateDownloads'] as bool? ?? true,
 );
 
 Map<String, dynamic> _$AppSettingsToJson(
@@ -140,4 +141,5 @@ Map<String, dynamic> _$AppSettingsToJson(
   'lyricsMultiPersonWordByWord': instance.lyricsMultiPersonWordByWord,
   'musixmatchLanguage': instance.musixmatchLanguage,
   'lastSeenVersion': instance.lastSeenVersion,
+  'deduplicateDownloads': instance.deduplicateDownloads,
 };