Rewrite email parser using ImapEngine, harden processing loop

Replace webklex/php-imap with directorytree/imapengine in the ticket
email parser. ImapEngine is pure PHP over sockets.

Parser improvements:
- Wrap per-message processing in try/catch so one malformed email
  can't abort the run; failures are flagged and logged with UID
- Query unseen + unflagged so previously-failed (flagged) messages
  are no longer re-processed on every cron run
- Skip vacation/auto-responder emails (RFC 3834) to prevent mail
  loops with the ticket auto-reply
- Cap messages per run (50) and attachment size (15MB); inline
  images over 2MB are stored as attachments instead of base64-embedded
  in ticket details
- Atomic lock file creation
- preg_quote() the ticket prefix in subject matching
- Dedupe CC watchers and exclude the sender
- Map webklex 'tls' encryption setting to STARTTLS for compatibility

NDR/DSN parsing now walks MIME parts via the underlying
zbateson parser instead of relying on attachment extraction.

plugins/vendor/symfony/mime/DependencyInjection/AddMimeTypeGuesserPass.php

@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\DependencyInjection;
+
+use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Reference;
+
+/**
+ * Registers custom mime types guessers.
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+class AddMimeTypeGuesserPass implements CompilerPassInterface
+{
+    public function process(ContainerBuilder $container): void
+    {
+        if (!$container->has('mime_types')) {
+            return;
+        }
+        $definition = $container->findDefinition('mime_types');
+        $id = null;
+        foreach ($container->findTaggedServiceIds('mime.mime_type_guesser', true) as $id => $attributes) {
+            $definition->addMethodCall('registerGuesser', [new Reference($id)]);
+        }
+        if (null !== $id) {
+            $definition->setPublic(true);
+        }
+    }
+}