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/Encoder/IdnAddressEncoder.php

@@ -0,0 +1,44 @@
+<?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\Encoder;
+
+/**
+ * An IDN email address encoder.
+ *
+ * Encodes the domain part of an address using IDN. This is compatible will all
+ * SMTP servers.
+ *
+ * Note: It leaves the local part as is. In case there are non-ASCII characters
+ * in the local part then it depends on the SMTP Server if this is supported.
+ *
+ * @author Christian Schmidt
+ */
+final class IdnAddressEncoder implements AddressEncoderInterface
+{
+    /**
+     * Encodes the domain part of an address using IDN.
+     */
+    public function encodeString(string $address): string
+    {
+        $i = strrpos($address, '@');
+        if (false !== $i) {
+            $local = substr($address, 0, $i);
+            $domain = substr($address, $i + 1);
+
+            if (preg_match('/[^\x00-\x7F]/', $domain)) {
+                $address = \sprintf('%s@%s', $local, idn_to_ascii($domain, \IDNA_DEFAULT | \IDNA_USE_STD3_RULES | \IDNA_CHECK_BIDI | \IDNA_CHECK_CONTEXTJ | \IDNA_NONTRANSITIONAL_TO_ASCII, \INTL_IDNA_VARIANT_UTS46));
+            }
+        }
+
+        return $address;
+    }
+}