feat(exceptions): add CancellationException for rejected cancellation

Signed-off-by: Vitor Mattos <vitor@php.rio>

Author: vitormattos

src/Exception/CancellationException.php

@@ -0,0 +1,15 @@
+<?php
+
+// SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+declare(strict_types=1);
+
+namespace LibreCodeCoop\NfsePHP\Exception;
+
+/**
+ * Thrown when the SEFIN gateway rejects an NFS-e cancellation request.
+ */
+class CancellationException extends GatewayException
+{
+}