fix(metadata): support extension custom search

test/maintainability_contracts_test.dart

@@ -173,6 +173,8 @@ void main() {
         ]),
         'songs',
       );
+      expect(extension.searchBehavior?.filterIdForKind('track'), 'songs');
+      expect(extension.searchBehavior?.filterIdForKind('album'), 'records');
       expect(
         HomeSearchProviderPolicy.displayFilterSelection(
           null,

test/platform_bridge_metadata_test.dart

@@ -104,4 +104,30 @@ void main() {
     });
     expect(results.single['provider_id'], 'selected-metadata');
   });
+
+  test('metadata custom search sends the manifest track filter', () async {
+    MethodCall? capturedCall;
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(backendChannel, (call) async {
+          capturedCall = call;
+          return jsonEncode([
+            {'id': 'track-1', 'name': 'Song', 'provider_id': 'custom-metadata'},
+          ]);
+        });
+
+    final results = await PlatformBridge.customSearchWithExtension(
+      'custom-metadata',
+      'Song Artist',
+      options: {'limit': 5, 'filter': 'songs'},
+    );
+
+    expect(capturedCall?.method, 'customSearchWithExtension');
+    final arguments = capturedCall?.arguments as Map<Object?, Object?>?;
+    expect(arguments, isNotNull);
+    expect(arguments?['extension_id'], 'custom-metadata');
+    expect(arguments?['query'], 'Song Artist');
+    expect(arguments?['options'], jsonEncode({'limit': 5, 'filter': 'songs'}));
+    expect(arguments?['request_id'], isA<String>());
+    expect(results.single['provider_id'], 'custom-metadata');
+  });
 }