Neural sparse query two-phase search processor's bwc test

[conggguan]

Description

Add BWC test for the Neural sparse query two-phase search processor.

Issues Resolved

Resolve #646.

Check List

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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[conggguan]

Can you please check why bwc are failing in CI for rolling upgrade? I've restarted them few times but it still failing, doesn't look like a flaky tests

Hi, I have made a test qa:rolling-upgrade:testAgainstOneThirdUpgradedCluster on a aws ec2, which based ubuntu 22 and JDK17.
The test branch is main and without this PR's code, and get a similar error log.

* What went wrong:
Execution failed for task ':qa:rolling-upgrade:testAgainstOneThirdUpgradedCluster'.
> `cluster{:qa:rolling-upgrade:neuralSearchBwcCluster-rolling}` failed to wait for cluster health yellow after 40 SECONDS
    IO error while waiting cluster
    503 Service Unavailable

Based this, I think the failed maybe caused by something besides my PR.

[yuye-aws]

Can you update the changelog?

[conggguan]

Can you update the changelog?

Sure, add a line in changelog.

[martin-gaievski]

please make a proper changelog entry - pr number and link are missing

[conggguan]

Okay, have added it to the change log.

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-777-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 b0b51284d29532ed67fcff745fee1d45653ce776
# Push it to GitHub
git push --set-upstream origin backport/backport-777-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-777-to-2.x.