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/Test/Constraint/EmailSubjectContains.php

@@ -0,0 +1,47 @@
+<?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\Test\Constraint;
+
+use PHPUnit\Framework\Constraint\Constraint;
+use Symfony\Component\Mime\Email;
+
+final class EmailSubjectContains extends Constraint
+{
+    public function __construct(
+        private readonly string $expectedSubjectValue,
+    ) {
+    }
+
+    public function toString(): string
+    {
+        return \sprintf('contains subject with value "%s"', $this->expectedSubjectValue);
+    }
+
+    protected function matches($other): bool
+    {
+        if (!$other instanceof Email) {
+            throw new \LogicException('Can only test a message subject on an Email instance.');
+        }
+
+        return str_contains((string) $other->getSubject(), $this->expectedSubjectValue);
+    }
+
+    protected function failureDescription($other): string
+    {
+        $message = 'The email subject '.$this->toString();
+        if ($other instanceof Email) {
+            $message .= \sprintf('. The subject was: "%s"', $other->getSubject() ?? '<empty>');
+        }
+
+        return $message;
+    }
+}