Fix the notification email theme for asynchronously dispatched emails

When the `\Symfony\Component\Mailer\Messenger\SendEmailMessage` is dispatched asynchronously, the email message is serialised and unserialised. The theme that was set on the `NotificationEmail` was not included in the serialisation, causing the
value to return back to the default after deserialisation.

Author: krisbuist

Mime/NotificationEmail.php

@@ -210,15 +210,20 @@ private function getExceptionAsString($exception): string
      */
     public function __serialize(): array
     {
-        return [$this->context, parent::__serialize()];
+        return [$this->context, $this->theme, parent::__serialize()];
     }
 
     /**
      * @internal
      */
     public function __unserialize(array $data): void
     {
-        [$this->context, $parentData] = $data;
+        if (3 === \count($data)) {
+            [$this->context, $this->theme, $parentData] = $data;
+        } else {
+            // Backwards compatibility for deserializing data structures that were serialized without the theme
+            [$this->context, $parentData] = $data;
+        }
 
         parent::__unserialize($parentData);
     }

Tests/Mime/NotificationEmailTest.php

@@ -46,6 +46,7 @@ public function testSerialize()
             ->importance(NotificationEmail::IMPORTANCE_HIGH)
             ->action('Bar', 'http://example.com/')
             ->context(['a' => 'b'])
+            ->theme('example')
         ));
         $this->assertEquals([
             'importance' => NotificationEmail::IMPORTANCE_HIGH,
@@ -57,6 +58,8 @@ public function testSerialize()
             'raw' => true,
             'a' => 'b',
         ], $email->getContext());
+
+        $this->assertSame('@email/example/notification/body.html.twig', $email->getHtmlTemplate());
     }
 
     public function testTheme()