Add fake 64_json string to test image response

Author: JanSuthacheeva

src/Testing/Responses/Fixtures/Images/CreateResponseFixture.php

@@ -9,6 +9,7 @@ final class CreateResponseFixture
         'data' => [
             [
                 'url' => 'https://openai.com/fake-image.png',
+                'b64_json' => 'fake-b64_json',
             ],
         ],
     ];