fix: enhance root node insertion logic to check for existing nodes with left attribute.

Author: terabytesoftw

src/NestedSetsBehavior.php

@@ -1070,7 +1070,10 @@ protected function beforeInsertNode(int $value, int $depth): void
      */
     protected function beforeInsertRootNode(): void
     {
-        if ($this->treeAttribute === false && $this->getOwner()::find()->roots()->exists()) {
+        if (
+            $this->treeAttribute === false &&
+            $this->getOwner()::find()->andWhere([$this->leftAttribute => 1])->exists()
+        ) {
             throw new Exception('Can not create more than one root when "treeAttribute" is false.');
         }