aqueduct ENG-1452-remove base64 encoding for artifact results when calling preview response

Describe your changes and why you are making these changes

remove the unnecessary steps of converting bytes array to base64 string in the preview response route After the code change, in the preview response route, the backend will package the artifact_results as a byte array and send in a multipart-data-form format as a response and being correctly received by the sdk as a post response.

Related issue number (if any)

Checklist before requesting a review

[ ] I have created a descriptive PR title. The PR title should complete the sentence "This PR...".
[ ] I have performed a self-review of my code.
[ ] I have included a small demo of the changes. For the UI, this would be a screenshot or a Loom video.
[ ] If this is a new feature, I have added unit tests and integration tests.
[ ] I have run the integration tests locally and they are passing.
[ ] I have run the linter script locally (See python3 scripts/run_linters.py -h for usage).
[ ] All features on the UI continue to work correctly.
[ ] Added one of the following CI labels:
- run_integration_test: Runs integration tests
- skip_integration_test: Skips integration tests (Should be used when changes are ONLY documentation/UI)