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/Header/Part/ParameterPart.php

@@ -0,0 +1,119 @@
+<?php
+/**
+ * This file is part of the ZBateson\MailMimeParser project.
+ *
+ * @license http://opensource.org/licenses/bsd-license.php BSD
+ */
+
+namespace ZBateson\MailMimeParser\Header\Part;
+
+use Psr\Log\LoggerInterface;
+use ZBateson\MbWrapper\MbWrapper;
+
+/**
+ * Represents a name/value parameter part of a header.
+ *
+ * @author Zaahid Bateson
+ */
+class ParameterPart extends NameValuePart
+{
+    /**
+     * @var string the RFC-1766 language tag if set.
+     */
+    protected ?string $language = null;
+
+    /**
+     * @var string charset of content if set.
+     */
+    protected ?string $charset = null;
+
+    /**
+     * @var int the zero-based index of the part if part of a 'continuation' in
+     *      an RFC-2231 split parameter.
+     */
+    protected ?int $index = null;
+
+    /**
+     * @var bool true if the part is an RFC-2231 encoded part, and the value
+     *      needs to be decoded.
+     */
+    protected bool $encoded = false;
+
+    /**
+     * @param HeaderPart[] $nameParts
+     */
+    public function __construct(
+        LoggerInterface $logger,
+        MbWrapper $charsetConverter,
+        array $nameParts,
+        ContainerPart $valuePart
+    ) {
+        parent::__construct($logger, $charsetConverter, $nameParts, $valuePart->children);
+    }
+
+    protected function getNameFromParts(array $parts) : string
+    {
+        $name = parent::getNameFromParts($parts);
+        if (\preg_match('~^\s*([^\*]+)\*(\d*)(\*)?$~', $name, $matches)) {
+            $name = $matches[1];
+            $this->index = ($matches[2] !== '') ? (int) ($matches[2]) : null;
+            $this->encoded = (($matches[2] === '') || !empty($matches[3]));
+        }
+        return $name;
+    }
+
+    protected function decodePartValue(string $value, ?string $charset = null) : string
+    {
+        if ($charset !== null) {
+            return $this->convertEncoding(\rawurldecode($value), $charset, true);
+        }
+        return $this->convertEncoding(\rawurldecode($value));
+    }
+
+    protected function getValueFromParts(array $parts) : string
+    {
+        $value = parent::getValueFromParts($parts);
+        if ($this->encoded && \preg_match('~^([^\']*)\'?([^\']*)\'?(.*)$~', $value, $matches)) {
+            $this->charset = (!empty($matches[1]) && !empty($matches[3])) ? $matches[1] : $this->charset;
+            $this->language = (!empty($matches[2])) ? $matches[2] : $this->language;
+            $ev = (empty($matches[3])) ? $matches[1] : $matches[3];
+            // only if it's not part of a SplitParameterPart
+            if ($this->index === null) {
+                // subsequent parts are decoded as a SplitParameterPart since only
+                // the first part are supposed to have charset/language fields
+                return $this->decodePartValue($ev, $this->charset);
+            }
+            return $ev;
+        }
+        return $value;
+    }
+
+    /**
+     * Returns the charset if the part is an RFC-2231 part with a charset set.
+     */
+    public function getCharset() : ?string
+    {
+        return $this->charset;
+    }
+
+    /**
+     * Returns the RFC-1766 (or subset) language tag, if the parameter is an
+     * RFC-2231 part with a language tag set.
+     *
+     * @return ?string the language if set, or null if not
+     */
+    public function getLanguage() : ?string
+    {
+        return $this->language;
+    }
+
+    public function isUrlEncoded() : bool
+    {
+        return $this->encoded;
+    }
+
+    public function getIndex() : ?int
+    {
+        return $this->index;
+    }
+}