Revert "mvc: UidField: allow reuse of UID 0 for system scope users"

This reverts commit 649bcb2186.
There is a bug in the old and new behaviour, but without a proper
test case this will not fix easily.  stable/26.7 doesn't use this
yet so it's sane to hotfix.

PR: https://github.com/opnsense/core/issues/10992
(cherry picked from commit 7bbd7a3c4a)

src/opnsense/mvc/app/models/OPNsense/Auth/FieldTypes/UidField.php

@@ -52,24 +52,18 @@ class UidField extends IntegerField
      */
     public function setValue($value)
     {
-        $in_scope = false;
-        $uids = [];
-
-        if ($this->fieldLoaded) {
+        if (empty((string)$this) && $this->fieldLoaded) {
+            $uids = [];
             foreach ($this->getParentModel()->user->iterateItems() as $user) {
                 $uids[] = $user->uid->asInt();
             }
-            $in_scope = !$this->getParentNode()->scope->isEqual('system');
-        }
-
-        if (!$this->isSet() || !$in_scope) {
-            for ($i = 2000; $in_scope; $i++) {
+            for ($i = 2000; true; $i++) {
                 if (!in_array($i, $uids)) {
                     parent::setValue((string)$i);
-                    return;
+                    break;
                 }
             }
-
+        } elseif (empty((string)$this)) {
             parent::setValue($value);
         }
     }
@@ -93,6 +87,7 @@ class UidField extends IntegerField
         $this->setValue(null);
     }
 
+
     /**
      * retrieve field validators for this field type
      * @return array returns Text/regex validator