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/Part/AbstractMultipartPart.php

@@ -0,0 +1,95 @@
+<?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\Part;
+
+use Symfony\Component\Mime\Header\Headers;
+
+/**
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+abstract class AbstractMultipartPart extends AbstractPart
+{
+    private ?string $boundary = null;
+    private array $parts = [];
+
+    public function __construct(AbstractPart ...$parts)
+    {
+        parent::__construct();
+
+        foreach ($parts as $part) {
+            $this->parts[] = $part;
+        }
+    }
+
+    /**
+     * @return AbstractPart[]
+     */
+    public function getParts(): array
+    {
+        return $this->parts;
+    }
+
+    public function getMediaType(): string
+    {
+        return 'multipart';
+    }
+
+    public function getPreparedHeaders(): Headers
+    {
+        $headers = parent::getPreparedHeaders();
+        $headers->setHeaderParameter('Content-Type', 'boundary', $this->getBoundary());
+
+        return $headers;
+    }
+
+    public function bodyToString(): string
+    {
+        $parts = $this->getParts();
+        $string = '';
+        foreach ($parts as $part) {
+            $string .= '--'.$this->getBoundary()."\r\n".$part->toString()."\r\n";
+        }
+        $string .= '--'.$this->getBoundary()."--\r\n";
+
+        return $string;
+    }
+
+    public function bodyToIterable(): iterable
+    {
+        $parts = $this->getParts();
+        foreach ($parts as $part) {
+            yield '--'.$this->getBoundary()."\r\n";
+            yield from $part->toIterable();
+            yield "\r\n";
+        }
+        yield '--'.$this->getBoundary()."--\r\n";
+    }
+
+    public function asDebugString(): string
+    {
+        $str = parent::asDebugString();
+        foreach ($this->getParts() as $part) {
+            $lines = explode("\n", $part->asDebugString());
+            $str .= "\n  └ ".array_shift($lines);
+            foreach ($lines as $line) {
+                $str .= "\n  |".$line;
+            }
+        }
+
+        return $str;
+    }
+
+    private function getBoundary(): string
+    {
+        return $this->boundary ??= strtr(base64_encode(random_bytes(6)), '+/', '-_');
+    }
+}