test(export): set creation date on comments in UserExportTest

Comment::jsonSerialize() now includes creation_time and throws when no
creation date is set, which broke both UserExportTest cases.

Signed-off-by: Jonas <jonas@freesources.org>
Assisted-by: ClaudeCode:claude-opus-5.5

Author: mejo-

tests/unit/Command/UserExportTest.php

@@ -98,6 +98,7 @@ public function getComment($id) {
 		$comment = new Comment();
 		$comment->setActor('users', 'admin');
 		$comment->setMessage('fake comment' . $id);
+		$comment->setCreationDateTime(new \DateTime('2026-01-01T00:00:00+00:00'));
 		return $comment;
 	}