fix: update user validation in events

Update the user validation method in both `onUserSaveBegin` and `onUserSaveEnd` events. The
previous validation was using `QUI::getUsers()->isUser($User)`, which was replaced with a check for
`$User` instance of `QUI\\Users\\User` for improved reliability.

src/QUI/ERP/Customer/EventHandler.php

@@ -198,7 +198,7 @@ protected static function readAttributesFromUserXML(string $file): array
 
     public static function onUserSaveBegin(QUI\Interfaces\Users\User $User): void
     {
-        if (!QUI::getUsers()->isUser($User)) {
+        if (!($User instanceof QUI\Users\User)) {
             return;
         }
 
@@ -238,7 +238,7 @@ public static function onUserSaveBegin(QUI\Interfaces\Users\User $User): void
      */
     public static function onUserSaveEnd(QUI\Interfaces\Users\User $User): void
     {
-        if (!QUI::getUsers()->isUser($User)) {
+        if (!($User instanceof QUI\Users\User)) {
             return;
         }