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/Part/MessagePart.php

@@ -0,0 +1,71 @@
+<?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\Part;
+
+use Symfony\Component\Mime\Message;
+use Symfony\Component\Mime\RawMessage;
+
+/**
+ * @final
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+class MessagePart extends DataPart
+{
+    public function __construct(
+        private RawMessage $message,
+    ) {
+        if ($message instanceof Message) {
+            $name = $message->getHeaders()->getHeaderBody('Subject').'.eml';
+        } else {
+            $name = 'email.eml';
+        }
+        parent::__construct('', $name);
+    }
+
+    public function getMediaType(): string
+    {
+        return 'message';
+    }
+
+    public function getMediaSubtype(): string
+    {
+        return 'rfc822';
+    }
+
+    public function getBody(): string
+    {
+        return $this->message->toString();
+    }
+
+    public function bodyToString(): string
+    {
+        return $this->getBody();
+    }
+
+    public function bodyToIterable(): iterable
+    {
+        return $this->message->toIterable();
+    }
+
+    public function __serialize(): array
+    {
+        return ['message' => $this->message];
+    }
+
+    public function __unserialize(array $data): void
+    {
+        $this->message = $data['message'] ?? $data["\0".self::class."\0message"];
+
+        $this->__construct($this->message);
+    }
+}