fix: align test namespace for Unit/Service/SignerElementsServiceTest.php

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

tests/php/Unit/Service/SignerElementsServiceTest.php

@@ -1,6 +1,9 @@
 <?php
 
 declare(strict_types=1);
+
+namespace OCA\Libresign\Tests\Unit\Service;
+
 /**
  * SPDX-FileCopyrightText: 2025 LibreCode coop and contributors
  * SPDX-License-Identifier: AGPL-3.0-or-later