YtSearchService: Gast-Zugang neben Baka-Auth unterstützen

Analog zu YtDownloadService: optionaler gast-Parameter, X-Guest-
Token statt Authorization, automatisches Token-Nachholen bei 401.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/services/yt_search_service_test.dart

@@ -5,6 +5,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:http/http.dart' as http;
 import 'package:http/testing.dart';
 import 'package:melo/services/baka_auth.dart';
+import 'package:melo/services/gast_zugang.dart';
 import 'package:melo/services/yt_search_service.dart';
 
 class _MemorySpeicher implements TokenSpeicher {
@@ -200,5 +201,61 @@ void main() {
       await dienst.suchen('zweite suche');
       expect(dienst.treffer.single.title, 'Zweites');
     });
+
+    test('Mit Gast-Token statt Anmeldung wird trotzdem gesucht', () async {
+      final speicher = _MemorySpeicher();
+      speicher.werte['guest_token'] = 'gast-xyz';
+      final gast = GastZugang(
+        client: MockClient((_) async => http.Response('', 500)),
+        speicher: speicher,
+      );
+      await gast.laden();
+
+      final dienst = YtSearchService(
+        auth: BakaAuth(
+            client: MockClient((_) async => http.Response('', 500)),
+            speicher: _MemorySpeicher()),
+        gast: gast,
+        client: MockClient((req) async {
+          expect(req.headers['X-Guest-Token'], 'gast-xyz');
+          expect(req.headers.containsKey('Authorization'), isFalse);
+          return http.Response('[]', 200);
+        }),
+      );
+
+      await dienst.suchen('roses');
+
+      expect(dienst.fehler, isNull);
+    });
+
+    test('401 bei Gast-Zugang holt automatisch einen neuen Gast-Token',
+        () async {
+      var tokenAufrufe = 0;
+      final speicher = _MemorySpeicher();
+      speicher.werte['guest_token'] = 'alter-token';
+      final gast = GastZugang(
+        client: MockClient((_) async {
+          tokenAufrufe++;
+          return http.Response(
+              jsonEncode({'guest_token': 'neuer-token'}), 200);
+        }),
+        speicher: speicher,
+      );
+      await gast.laden();
+
+      final dienst = YtSearchService(
+        auth: BakaAuth(
+            client: MockClient((_) async => http.Response('', 500)),
+            speicher: _MemorySpeicher()),
+        gast: gast,
+        client: MockClient((_) async => http.Response('Unauthorized', 401)),
+      );
+
+      await dienst.suchen('roses');
+      await Future<void>.delayed(Duration.zero);
+
+      expect(dienst.fehler, 'Gast-Zugang abgelaufen — bitte erneut versuchen');
+      expect(tokenAufrufe, 1);
+    });
   });
 }