added guardian letters with chat and multiple answer functionalities

lib/api/marianumconnect/queries/auth_guardian/guardian_login_exception.dart

@@ -84,21 +84,9 @@ class GuardianLoginException extends AppException {
           'Bitte versuche es später erneut.',
   };
 
-  /// Accepts the documented JSON body (`{"error": …, "attemptsLeft": …}`) as
-  /// well as the plain-text `Fehler: <code>` the server's generic error
-  /// handler produces.
   static (String?, int?) _parseBody(Object? data) {
-    if (data is Map) {
-      final attempts = data['attemptsLeft'];
-      return (data['error'] as String?, attempts is int ? attempts : null);
-    }
-    if (data is String) {
-      final code = data.startsWith('Fehler: ')
-          ? data.substring('Fehler: '.length)
-          : data;
-      return (code.trim(), null);
-    }
-    return (null, null);
+    final attempts = data is Map ? data['attemptsLeft'] : null;
+    return (marianumConnectErrorCode(data), attempts is int ? attempts : null);
   }
 
   static GuardianLoginError? _errorFor(String? code, int status) =>