simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/search/search_files.dart

@@ -1,4 +1,3 @@
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
@@ -28,9 +27,7 @@ class SearchFiles {
         'Files search failed with ${response.statusCode}: ${response.body}',
       );
     }
-    final decoded = jsonDecode(response.body) as Map<String, dynamic>;
-    final ocs = decoded['ocs'] as Map<String, dynamic>;
-    final data = ocs['data'] as Map<String, dynamic>;
+    final data = NextcloudOcs.decode(response.body)['data'] as Map<String, dynamic>;
     return SearchFilesResponse.fromJson(data);
   }
 }