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/HeaderInterface.php

@@ -0,0 +1,61 @@
+<?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;
+
+/**
+ * A MIME Header.
+ *
+ * @author Chris Corbyn
+ */
+interface HeaderInterface
+{
+    /**
+     * Sets the body.
+     *
+     * The type depends on the Header concrete class.
+     */
+    public function setBody(mixed $body): void;
+
+    /**
+     * Gets the body.
+     *
+     * The return type depends on the Header concrete class.
+     */
+    public function getBody(): mixed;
+
+    public function setCharset(string $charset): void;
+
+    public function getCharset(): ?string;
+
+    public function setLanguage(string $lang): void;
+
+    public function getLanguage(): ?string;
+
+    public function getName(): string;
+
+    public function setMaxLineLength(int $lineLength): void;
+
+    public function getMaxLineLength(): int;
+
+    /**
+     * Gets this Header rendered as a compliant string.
+     */
+    public function toString(): string;
+
+    /**
+     * Gets the header's body, prepared for folding into a final header value.
+     *
+     * This is not necessarily RFC 2822 compliant since folding white space is
+     * not added at this stage (see {@link toString()} for that).
+     */
+    public function getBodyAsString(): string;
+}