fix: assign correct return types

src/BusProNet/ApiClient.php

@@ -83,7 +83,7 @@ class ApiClient
     /**
      * @throws ApiClientException
      */
-    public function register(RegistrationData $registrationData): Notification
+    public function register(RegistrationData $registrationData): ContactFormResponse|Notification
     {
         $data = [
             'user' => $this->config['bpn_username'],