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/File.php

@@ -0,0 +1,51 @@
+<?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\MimeTypes;
+
+/**
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+class File
+{
+    private static MimeTypes $mimeTypes;
+
+    public function __construct(
+        private string $path,
+        private ?string $filename = null,
+    ) {
+    }
+
+    public function getPath(): string
+    {
+        return $this->path;
+    }
+
+    public function getContentType(): string
+    {
+        $ext = strtolower(pathinfo($this->path, \PATHINFO_EXTENSION));
+        self::$mimeTypes ??= new MimeTypes();
+
+        return self::$mimeTypes->getMimeTypes($ext)[0] ?? 'application/octet-stream';
+    }
+
+    public function getSize(): int
+    {
+        return filesize($this->path);
+    }
+
+    public function getFilename(): string
+    {
+        return $this->filename ??= basename($this->getPath());
+    }
+}