Add isExcludedFromInvoicing method to departments_o

- Introduced `isExcludedFromInvoicing` to check if a department is excluded from invoicing.
- Ensured selected department validation before retrieving exclusion status.

services/nginx/app/objects/departments_o.php

@@ -400,4 +400,15 @@ class departments_o extends db
         // Remove duplicate products
         return array_unique($products, SORT_REGULAR);
     }
+
+    /**
+     * If the department is excluded from invoicing
+     * @return bool
+     * @throws Exception If the department is not selected
+     */
+    public function isExcludedFromInvoicing(): bool
+    {
+        self::requireSelected();
+        return $this->variables->getVariable('exclude_from_invoicing') === true;
+    }
 }