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/Base64ContentEncoder.php

@@ -0,0 +1,45 @@
+<?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;
+
+use Symfony\Component\Mime\Exception\RuntimeException;
+
+/**
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+final class Base64ContentEncoder extends Base64Encoder implements ContentEncoderInterface
+{
+    public function encodeByteStream($stream, int $maxLineLength = 0): iterable
+    {
+        if (!\is_resource($stream)) {
+            throw new \TypeError(\sprintf('Method "%s" takes a stream as a first argument.', __METHOD__));
+        }
+
+        $filter = stream_filter_append($stream, 'convert.base64-encode', \STREAM_FILTER_READ, [
+            'line-length' => 0 >= $maxLineLength || 76 < $maxLineLength ? 76 : $maxLineLength,
+            'line-break-chars' => "\r\n",
+        ]);
+        if (!\is_resource($filter)) {
+            throw new RuntimeException('Unable to set the base64 content encoder to the filter.');
+        }
+
+        while (!feof($stream)) {
+            yield fread($stream, 16372);
+        }
+        stream_filter_remove($filter);
+    }
+
+    public function getName(): string
+    {
+        return 'base64';
+    }
+}