Experimental dataset JSON GET API doesn't return multi-valued fields as arrays

What steps does it take to reproduce the issue?

1. Create dataset with multi-valued field, e.g. subject in the citation block
2. Use experimental GET API to retrieve dataset https://guides.dataverse.org/en/latest/developers/dataset-semantic-metadata-api.html#get-dataset-metadata e.g. GET /api/datasets/42/metadata

• What happens?

The multi-valued field values are not returned as arrays, e.g. 'subject': 'Medicine, Health and Life Sciences'

Side note, the non-experimental API https://guides.dataverse.org/en/latest/api/native-api.html#get-json-representation-of-a-dataset does not have this bug: {'typeName': 'subject', 'multiple': True, 'typeClass': 'controlledVocabulary', 'value': ['Medicine, Health and Life Sciences']}

• What did you expect to happen?

The multi-valued field values to be returned as arrays, e.g. 'subject': ['Medicine, Health and Life Sciences']

Which version of Dataverse are you using?

v5.13

Any related open or closed issues to this bug report?

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Screenshots:

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Start with the experimental GET API at /api/datasets/42/metadata and compare its multi-valued field response with the native API representation described in the linked documentation. Reproduce the subject-field case and verify that multi-valued fields are returned as arrays; an open pull request is already linked to this issue.