[TEST][BUG] Prometheus query API returns 500 error for scalar result type

[qianheng-aws]

What is the bug?
When using Prometheus query API with scalar functions like scalar(count(up)), the API returns a 500 internal server error instead of the expected result.

How can one reproduce the bug?

1. Add a Prometheus data source
2. Run a PROMQL query that returns a scalar result type:

curl -X 'POST' -H 'Content-Type:application/json' --data-binary '{
  "query": "scalar(count(up))",
  "language": "PROMQL",
  "options": {
    "queryType": "instant",
    "time": "1773874502"
  }
}' 'localhost:9200/_plugins/_directquery/_query/my_prometheus'

What is the expected behavior?
The query should return the scalar result without error.

What is your host/environment?
OpenSearch 2.19

[github-actions]

Possible Duplicate

Found 1 possible duplicate issue(s):

• [BUG] Prometheus query API does not support scalar #5251 — [BUG] Prometheus query API does not support scalar

If this is not a duplicate:

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[ahkcs]

Closing because of duplicate issue