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/zbateson/mail-mime-parser/src/Parser/AbstractParserService.php

@@ -0,0 +1,75 @@
+<?php
+/**
+ * This file is part of the ZBateson\MailMimeParser project.
+ *
+ * @license http://opensource.org/licenses/bsd-license.php BSD
+ */
+
+namespace ZBateson\MailMimeParser\Parser;
+
+use ZBateson\MailMimeParser\Parser\Proxy\ParserPartProxyFactory;
+
+/**
+ * Provides basic implementations for:
+ * - IParser::setParserManager
+ * - IParser::getParserMessageProxyFactory (returns $this->parserMessageProxyFactory
+ *   which can be set via the default constructor)
+ * - IParser::getParserPartProxyFactory (returns $this->parserPartProxyFactory
+ *   which can be set via the default constructor)
+ *
+ * @author Zaahid Bateson
+ */
+abstract class AbstractParserService implements IParserService
+{
+    /**
+     * @var ParserPartProxyFactory the parser's message proxy factory service
+     *      responsible for creating an IMessage part wrapped in a
+     *      ParserPartProxy.
+     */
+    protected ParserPartProxyFactory $parserMessageProxyFactory;
+
+    /**
+     * @var ParserPartProxyFactory the parser's part proxy factory service
+     *      responsible for creating IMessagePart parts wrapped in a
+     *      ParserPartProxy.
+     */
+    protected ParserPartProxyFactory $parserPartProxyFactory;
+
+    /**
+     * @var PartBuilderFactory Service for creating PartBuilder objects for new
+     *      children.
+     */
+    protected PartBuilderFactory $partBuilderFactory;
+
+    /**
+     * @var ParserManagerService the ParserManager, which should call setParserManager
+     *      when the parser is added.
+     */
+    protected ParserManagerService $parserManager;
+
+    public function __construct(
+        ParserPartProxyFactory $parserMessageProxyFactory,
+        ParserPartProxyFactory $parserPartProxyFactory,
+        PartBuilderFactory $partBuilderFactory
+    ) {
+        $this->parserMessageProxyFactory = $parserMessageProxyFactory;
+        $this->parserPartProxyFactory = $parserPartProxyFactory;
+        $this->partBuilderFactory = $partBuilderFactory;
+    }
+
+    public function setParserManager(ParserManagerService $pm) : static
+    {
+        $this->parserManager = $pm;
+        return $this;
+    }
+
+    public function getParserMessageProxyFactory() : ParserPartProxyFactory
+    {
+        return $this->parserMessageProxyFactory;
+    }
+
+    public function getParserPartProxyFactory() : ParserPartProxyFactory
+    {
+        return $this->parserPartProxyFactory;
+    }
+}