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/MailboxHeader.php

@@ -0,0 +1,85 @@
+<?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;
+
+use Symfony\Component\Mime\Address;
+use Symfony\Component\Mime\Exception\RfcComplianceException;
+
+/**
+ * A Mailbox MIME Header for something like Sender (one named address).
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+final class MailboxHeader extends AbstractHeader
+{
+    private Address $address;
+
+    public function __construct(string $name, Address $address)
+    {
+        parent::__construct($name);
+
+        $this->setAddress($address);
+    }
+
+    /**
+     * @param Address $body
+     *
+     * @throws RfcComplianceException
+     */
+    public function setBody(mixed $body): void
+    {
+        $this->setAddress($body);
+    }
+
+    /**
+     * @throws RfcComplianceException
+     */
+    public function getBody(): Address
+    {
+        return $this->getAddress();
+    }
+
+    /**
+     * @throws RfcComplianceException
+     */
+    public function setAddress(Address $address): void
+    {
+        $this->address = $address;
+    }
+
+    public function getAddress(): Address
+    {
+        return $this->address;
+    }
+
+    public function getBodyAsString(): string
+    {
+        $str = $this->address->getEncodedAddress();
+        if ($name = $this->address->getName()) {
+            $str = $this->createPhrase($this, $name, $this->getCharset(), true).' <'.$str.'>';
+        }
+
+        return $str;
+    }
+
+    /**
+     * Redefine the encoding requirements for an address.
+     *
+     * All "specials" must be encoded as the full header value will not be quoted
+     *
+     * @see RFC 2822 3.2.1
+     */
+    protected function tokenNeedsEncoding(string $token): bool
+    {
+        return preg_match('/[()<>\[\]:;@\,."]/', $token) || parent::tokenNeedsEncoding($token);
+    }
+}