Fix User class: enhance GroupsList property to safely parse UserGroup enums

FoodsharingSiegen.Contracts/Entity/User.cs

@@ -42,7 +42,12 @@ namespace FoodsharingSiegen.Contracts.Entity
             {
                 if (string.IsNullOrWhiteSpace(Groups)) return new List<UserGroup>();
                 var stringList = Groups.Split(",");
-                var enumList = stringList.Where(x => !string.IsNullOrWhiteSpace(x)).Select(Enum.Parse<UserGroup>).ToList();
+                var enumList = stringList
+                    .Where(x => !string.IsNullOrWhiteSpace(x))
+                    .Select(x => Enum.TryParse<UserGroup>(x.Trim(), out var result) ? result : (UserGroup?)null)
+                    .Where(x => x.HasValue)
+                    .Select(x => x!.Value)
+                    .ToList();
                 return enumList;
             }
             set => Groups = string.Join(",", value);