[BUG] GetRequest API is not returning featureAttributes as expected

What is the bug?

After creating index using IndexRequest, it returns the following response containing featureAttributes:aggregation:sum with "field": "value".

While retrieving the same index using GetRequest the same field is coming as null. (See under featureAttributes:aggregation.sum)

How can one reproduce the bug?

1. Create index with the mapping

IndexRequest<AnomalyDetector> indexRequest = new IndexRequest.Builder<AnomalyDetector>()
            .index(ANOMALY_DETECTORS_INDEX)
            .document(detector)
            .build(); 

2. Get Index

getResponse = client.get(s -> s.index(ANOMALY_DETECTORS_INDEX).id(detectorId), AnomalyDetector.class);

What is the expected behavior?

GetRequest API should return the attributes which were created using IndexRequest

What is your host/environment?

Operating system, version.

Do you have any screenshots?

If applicable, add screenshots to help explain your problem.

Do you have any additional context?

Add any other context about the problem.

Start with the anomaly-detectors.json mapping and the IndexRequest/GetRequest examples in the issue, then reproduce the round trip for an AnomalyDetector using the OpenSearch Java client. Compare the created response with the retrieved response, focusing on featureAttributes:aggregation.sum; done means GetRequest returns the attributes created by IndexRequest.