[Backport 2.x] Fix MLModelTool returns null if the response of LLM is a pure json object

[qianheng-aws]

Description

Backport 007b914 from #2655

And fix compiling failure by removing java21 API. (main use java 21 while 2.x use java17)

Issues Resolved

#2654

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[dhrubo-os]

Let's not diverge the code. Let's apply this into main too. I meant:

               Map<String, ?> dataAsMap = mlModelOutputs.get(0).getMlModelTensors().get(0).getDataAsMap();
             

[qianheng-aws]

Let's not diverge the code. Let's apply this into main too. I meant:

               Map<String, ?> dataAsMap = mlModelOutputs.get(0).getMlModelTensors().get(0).getDataAsMap();
             

Raised this PR #2682 to avoid diverge on main branch.

[b4sjoo]

@dhrubo-os What about backporting this PR change into 2.13, 2.15, and better 2.14 (although it does not have patch release but we should keep code base as consistent as possible).

[dhrubo-os]

@dhrubo-os What about backporting this PR change into 2.13, 2.15, and better 2.14 (although it does not have patch release but we should keep code base as consistent as possible).

Yup we should do that.

[b4sjoo]

@dhrubo-os Remember me if I was wrong, did we come into a conclusion that if we need to backport this PR to 2.11 or not?

[dhrubo-os]

@dhrubo-os Remember me if I was wrong, did we come into a conclusion that if we need to backport this PR to 2.11 or not?

We introduced Agent feature from 2.13: https://opensearch.org/docs/latest/ml-commons-plugin/agents-tools/index/