Add search backpressure service check for query group tasks

[kaushalmahi12]

Description

This change is to address the bug when search backpressure service is gathering the node stats and there are QueryGroupTasks in the system which doesn't have the queryGroupId set. In this case there will be a NPE thrown for node stats due to this line.

But this will only happen for 2.x versions since they don't have the scroll API changes for setting the queryGroupId #17169. We had to remove the API change because of the backward compatibility failure which was blocking the merge but later maintainers removed the check for internalApi annotated classes since they are not meant to be used outside of opensearch core.

I will also add that change in 2.x subsequently.

Related Issues

Resolves #17518 (comment)

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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