Workaround doctrine serialisation issue in integration tests

Doctrine uses a default ProxyClassNameResolver but it is implemented
as an anonymous class and can not be serialised. This commit adds a custom
ProxyClassNameResolver that copies the anonymous one so that we avoid using the
anonymous fallback.
[MAILPOET-3658]

lib/Doctrine/ProxyClassNameResolver.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace MailPoet\Doctrine;
+
+use MailPoetVendor\Doctrine\Persistence\Mapping\ProxyClassNameResolver as IProxyClassNameResolver;
+
+/**
+ * This is exact copy of an anonymous class from \MailPoetVendor\Doctrine\Persistence\Mapping\AbstractClassMetadataFactory
+ * We need to use a non-anonymous class so that it is serializable within integration tests
+ * @see https://github.com/doctrine/persistence/blob/2.2.x/lib/Doctrine/Persistence/Mapping/AbstractClassMetadataFactory.php#L516-L536
+ */
+class ProxyClassNameResolver implements IProxyClassNameResolver {
+
+  /**
+   * @template T
+   * @return class-string<T>
+   */
+  public function resolveClassName(string $className) : string {
+    $pos = \strrpos($className, '\\' . \MailPoetVendor\Doctrine\Persistence\Proxy::MARKER . '\\');
+    if ($pos === \false) {
+      /** @var class-string<T> */
+      return $className;
+    }
+    /** @var class-string<T> */
+    return \substr($className, $pos + \MailPoetVendor\Doctrine\Persistence\Proxy::MARKER_LENGTH + 2);
+  }
+}

lib/Doctrine/TablePrefixMetadataFactory.php

@@ -22,6 +22,7 @@ class TablePrefixMetadataFactory extends ClassMetadataFactory {
       throw new \RuntimeException('DB table prefix not initialized');
     }
     $this->prefix = Env::$dbPrefix;
+    $this->setProxyClassNameResolver(new ProxyClassNameResolver());
   }
 
   public function getMetadataFor($className) {