[FEATURE]Improve test coverage for HttpJsonConnectorExecutor.java

[dhrubo-os]

We need to improve test coverage in our repo.

This issue will target only to improve test coverage for HttpJsonConnectorExecutor.java

From this url we can see where test coverage is missing for this class: https://app.codecov.io/gh/opensearch-project/ml-commons/blob/2.x/ml-algorithms%2Fsrc%2Fmain%2Fjava%2Forg%2Fopensearch%2Fml%2Fengine%2Falgorithms%2Fremote%2FHttpJsonConnectorExecutor.java

[TrungBui59]

@dhrubo-os Can I take this issue?

[dhrubo-os]

Sure, assigned the issue to you

[TrungBui59]

@dhrubo-os can you give some idea of some of tests needed? I am quite clueless on how many tests needed and what I should test