Add PdfSignedMessage and its serializer

Added a new class, PdfSignedMessage, to handle received signed PDF messages. Also, added a serializer for this class, PdfSignedMessageSerializer, for use with messaging. Furthermore, comment documentation has been added to RequestPdfSignMessage and its serializer for better clarity. Updated unit tests are also included.

src/Bundle/ChillDocStoreBundle/Service/Signature/Driver/BaseSigner/PdfSignedMessageHandler.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Chill\DocStoreBundle\Service\Signature\Driver\BaseSigner;
+
+use Psr\Log\LoggerInterface;
+use Symfony\Component\Messenger\Handler\MessageHandlerInterface;
+
+final readonly class PdfSignedMessageHandler implements MessageHandlerInterface
+{
+    /**
+     * log prefix
+     */
+    private const P = '[pdf signed message] ';
+
+    public function __construct(
+        private LoggerInterface $logger,
+    )
+    {
+    }
+
+    public function __invoke(PdfSignedMessage $message): void
+    {
+        $this->logger->info(self::P."a message is received", ['signaturedId' => $message->signatureId]);
+    }
+}