simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/talk/chat/get_chat.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 'get_chat_params.dart';
 import 'get_chat_response.dart';
@@ -15,10 +14,8 @@ class GetChat extends TalkApi<GetChatResponse> {
     : super('v1/chat/$chatToken', null, getParameters: params.toJson());
 
   @override
-  GetChatResponse assemble(String raw) {
-    final decoded = jsonDecode(raw) as Map<String, dynamic>;
-    return GetChatResponse.fromJson(decoded['ocs'] as Map<String, dynamic>);
-  }
+  GetChatResponse assemble(String raw) =>
+      GetChatResponse.fromJson(NextcloudOcs.decode(raw));
 
   @override
   Future<Response> request(

lib/api/marianumcloud/talk/chat/long_poll_chat.dart

@@ -1,5 +1,4 @@
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
@@ -57,8 +56,7 @@ class LongPollChat {
     final status = response.statusCode;
     if (status == 304) return null;
     if (status >= 200 && status < 300) {
-      final decoded = jsonDecode(response.body) as Map<String, dynamic>;
-      return GetChatResponse.fromJson(decoded['ocs'] as Map<String, dynamic>)
+      return GetChatResponse.fromJson(NextcloudOcs.decode(response.body))
         ..headers = response.headers;
     }
     throw ServerException(