[query assist] update api handler to accommodate new ml-commons config response

[joshuali925]

Description

update api handler to accommodate new ml-commons config response

Issues Resolved

addresses breaking change from https://github.com/opensearch-project/ml-commons/pull/2710

Check List

• [ ] New functionality includes testing.
  • [ ] All tests pass, including unit test, integration test and doctest
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
  • [ ] New functionality has user manual doc added
• [ ] Commits are signed per the DCO using --signoff

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