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/HeaderParserService.php

@@ -0,0 +1,67 @@
+<?php
+/**
+ * This file is part of the ZBateson\MailMimeParser project.
+ *
+ * @license http://opensource.org/licenses/bsd-license.php BSD
+ */
+
+namespace ZBateson\MailMimeParser\Parser;
+
+use Psr\Log\LogLevel;
+use ZBateson\MailMimeParser\Message\PartHeaderContainer;
+
+/**
+ * Reads headers from an input stream, adding them to a PartHeaderContainer.
+ *
+ * @author Zaahid Bateson
+ */
+class HeaderParserService
+{
+    /**
+     * Ensures the header isn't empty and contains a colon separator character,
+     * then splits it and adds it to the passed PartHeaderContainer.
+     *
+     * @param int $offset read offset for error reporting
+     * @param string $header the header line
+     * @param PartHeaderContainer $headerContainer the container
+     */
+    private function addRawHeaderToPart(int $offset, string $header, PartHeaderContainer $headerContainer) : static
+    {
+        if ($header !== '') {
+            if (\strpos($header, ':') !== false) {
+                $a = \explode(':', $header, 2);
+                $headerContainer->add($a[0], \trim($a[1]));
+            } else {
+                $headerContainer->addError(
+                    "Invalid header found at offset: $offset",
+                    LogLevel::ERROR
+                );
+            }
+        }
+        return $this;
+    }
+
+    /**
+     * Reads header lines up to an empty line, adding them to the passed
+     * PartHeaderContainer.
+     *
+     * @param resource $handle The resource handle to read from.
+     * @param PartHeaderContainer $container the container to add headers to.
+     */
+    public function parse($handle, PartHeaderContainer $container) : static
+    {
+        $header = '';
+        do {
+            $offset = \ftell($handle);
+            $line = MessageParserService::readLine($handle);
+            if ($line === false || $line === '' || $line[0] !== "\t" && $line[0] !== ' ') {
+                $this->addRawHeaderToPart($offset, $header, $container);
+                $header = '';
+            } else {
+                $line = "\r\n" . $line;
+            }
+            $header .= \rtrim($line, "\r\n");
+        } while ($header !== '');
+        return $this;
+    }
+}