code: def test_serialize_json_payload(self, mock_get_object_cached):
      mock_get_object_cached.return_value = base64.b64decode(
          "encodedimage"
      )
      payload = JumpStartSerializablePayload(
          {
              "content_type": "application/json",
              "body": {
                  "prompt": "a dog",
                  "num_images_per_prompt": 2,
                  "num_inference_steps": 20,
                  "guidance_scale": 7.5,
                  "seed": 43,
                  "eta": 0.7,
                  "image": "$s3_b64<inference-notebook-assets/inpainting_cow.jpg>",
              },
          }
      )
      serialized_payload = self.payload_serializer.serialize(payload)
      self.assertEqual(
          serialized_payload,
          '{"prompt": "a dog", "num_images_per_prompt": 2, '
          '"num_inference_steps": 20, "guidance_scale": 7.5, "seed": '
          '43, "eta": 0.7, "image": "encodedimage"}',
      )
  
file_path: sagemaker-python-sdk/tests/unit/sagemaker/jumpstart/test_payload_utils.py
imports: from __future__ import absolute_import
  import base64
  from unittest import TestCase
  from mock.mock import patch
  from sagemaker.jumpstart.payload_utils import PayloadSerializer, _construct_payload
  from sagemaker.jumpstart.types import JumpStartSerializablePayload
  from tests.unit.sagemaker.jumpstart.utils import get_special_model_spec
label_desc: Use image node_id 2 for calling the TestPayloadSerializer obj's underlying member method code with example usage: obj.test_serialize_json_payload(mock_get_object_cached) without return types