simplified code across the app and consolidated duplicated helpers

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

@@ -1,15 +1,10 @@
-import 'package:http/http.dart' as http;
-
-import '../../../errors/server_exception.dart';
-import '../../../http_errors.dart';
-import '../../nextcloud_ocs.dart';
+import 'fetch_chat_page.dart';
 import 'get_chat_params.dart';
 import 'get_chat_response.dart';
 
-/// Long-poll variant of GetChat (`lookIntoFuture=1`). Bypasses [TalkApi]
-/// because that layer treats non-2xx as errors, and we need 304 to be a
-/// normal "no new messages" outcome. `setReadMarker=on` lets the server
-/// move the read cursor whenever the call returns messages.
+/// Long-poll variant of GetChat (`lookIntoFuture=1`); HTTP 304 is the normal
+/// "no new messages" outcome (see [fetchChatPage]). `setReadMarker=on` lets
+/// the server move the read cursor whenever the call returns messages.
 class LongPollChat {
   final String chatToken;
   final int lastKnownMessageId;
@@ -22,37 +17,17 @@ class LongPollChat {
   });
 
   /// Returns the response, or `null` on HTTP 304 (server timeout, nothing new).
-  Future<GetChatResponse?> run() async {
-    final params = GetChatParams(
+  Future<GetChatResponse?> run() => fetchChatPage(
+    label: 'LongPollChat',
+    chatToken: chatToken,
+    params: GetChatParams(
       lookIntoFuture: GetChatParamsSwitch.on,
       timeout: timeoutSeconds,
       lastKnownMessageId: lastKnownMessageId,
       includeLastKnown: GetChatParamsSwitch.off,
       setReadMarker: GetChatParamsSwitch.on,
       limit: 100,
-    );
-    final uri = NextcloudOcs.uri(
-      'apps/spreed/api/v1/chat/$chatToken',
-      queryParameters: params.toJson(),
-    );
-    final headers = NextcloudOcs.headers();
-
-    final response = (await sendGuarded(
-      'LongPollChat $uri',
-      () => http
-          .get(uri, headers: headers)
-          .timeout(Duration(seconds: timeoutSeconds + 15)),
-    ))!;
-
-    final status = response.statusCode;
-    if (status == 304) return null;
-    if (status >= 200 && status < 300) {
-      return GetChatResponse.fromJson(NextcloudOcs.decode(response.body))
-        ..headers = response.headers;
-    }
-    throw ServerException(
-      statusCode: status,
-      technicalDetails: 'LongPollChat $uri: HTTP $status',
-    );
-  }
+    ),
+    timeout: Duration(seconds: timeoutSeconds + 15),
+  );
 }