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/Header/UnstructuredHeader.php

@@ -0,0 +1,66 @@
+<?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\Header;
+
+/**
+ * A Simple MIME Header.
+ *
+ * @author Chris Corbyn
+ */
+class UnstructuredHeader extends AbstractHeader
+{
+    private string $value;
+
+    public function __construct(string $name, string $value)
+    {
+        parent::__construct($name);
+
+        $this->setValue($value);
+    }
+
+    /**
+     * @param string $body
+     */
+    public function setBody(mixed $body): void
+    {
+        $this->setValue($body);
+    }
+
+    public function getBody(): string
+    {
+        return $this->getValue();
+    }
+
+    /**
+     * Get the (unencoded) value of this header.
+     */
+    public function getValue(): string
+    {
+        return $this->value;
+    }
+
+    /**
+     * Set the (unencoded) value of this header.
+     */
+    public function setValue(string $value): void
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * Get the value of this header prepared for rendering.
+     */
+    public function getBodyAsString(): string
+    {
+        return $this->encodeWords($this, $this->value);
+    }
+}