Fix: Prevent duplicate usernames across all user states

Changes:
- Updated username duplicate validation to consider all existing users.
- Prevented reuse of usernames belonging to deleted/disabled accounts.
- Fixed authentication conflicts caused by duplicate usernames.

Fixes #1809

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/utilities/Validator.cpp

@@ -116,15 +116,13 @@ bool util::isPasswordValid(const std::string& password)
  *     usersMap - map of user objects keyed by identifier
  * Returns:
  *     bool - true if the username is already in use by an active user, false otherwise
- * Notes:
- *     - Only considers users with state util::State::ACTIVE
  */
 bool util::isUsernameDuplicate(const std::string& username, const util::Map<std::string, User*>& usersMap)
 {
     int index = usersMap.findIf(
         [&](const std::string&, User* user)
         {
-            return (user->getUserName() == username && user->getState() == util::State::ACTIVE);
+            return (user->getUserName() == username);
         }
     );
     return index != -1;