simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/talk/create_room/create_room.dart

@@ -1,8 +1,7 @@
-import 'dart:convert';
-
 import 'package:http/http.dart' as http;
 import 'package:http/http.dart';
 
+import '../../nextcloud_ocs.dart';
 import '../talk_api.dart';
 import 'create_room_params.dart';
 import 'create_room_response.dart';
@@ -13,10 +12,8 @@ class CreateRoom extends TalkApi<CreateRoomResponse> {
   CreateRoom(this.params) : super('v4/room', params);
 
   @override
-  CreateRoomResponse assemble(String raw) {
-    final decoded = jsonDecode(raw) as Map<String, dynamic>;
-    return CreateRoomResponse.fromJson(decoded['ocs'] as Map<String, dynamic>);
-  }
+  CreateRoomResponse assemble(String raw) =>
+      CreateRoomResponse.fromJson(NextcloudOcs.decode(raw));
 
   @override
   Future<Response>? request(