Create separate SourceLookup instance per segment slice in SignificantTextAggregatorFactory

[jed326]

Description

PR contains 2 changes in separate commits:

1. Remove flakey assertion from SearchTimeoutIT.
2. Make SourceLookup thread safe in SignificantTextAggregatorFactory by creating a new SourceLookup for each aggregator.

Related Issues

Resolves #8790
Resolves #8509

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
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

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.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/20628/
• CommitID: c8b380f
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/20629/
• CommitID: 82b71fc

[codecov]

Codecov Report

Merging #8807 (0e8e1f9) into main (5d78de6) will increase coverage by 0.18%.
The diff coverage is 79.31%.

@@             Coverage Diff              @@
##               main    #8807      +/-   ##
============================================
+ Coverage     70.98%   71.17%   +0.18%     
- Complexity    57166    57255      +89     
============================================
  Files          4757     4757              
  Lines        269823   269832       +9     
  Branches      39474    39478       +4     
============================================
+ Hits         191536   192050     +514     
+ Misses        62158    61572     -586     
- Partials      16129    16210      +81     

Files Changed	Coverage Δ
...org/opensearch/index/seqno/ReplicationTracker.java	68.65% <77.77%> (-0.62%)	⬇️
.../replication/checkpoint/ReplicationCheckpoint.java	87.71% <100.00%> (+2.00%)	⬆️
...bucket/terms/SignificantTextAggregatorFactory.java	92.47% <100.00%> (-0.09%)	⬇️

... and 464 files with indirect coverage changes

[jed326]

@sohami @reta @andrross could you please help review this PR? Thanks!

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/20830/
• CommitID: 5f319b0
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testStartReplicaAfterPrimaryIndexesDocs
      1 org.opensearch.cluster.allocation.AwarenessAllocationIT.testThreeZoneOneReplicaWithForceZoneValueAndLoadAwareness

• URL: https://build.ci.opensearch.org/job/gradle-check/20827/
• CommitID: d53d0c2
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/20832/
• CommitID: 8ae9393

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/20869/
• CommitID: 8467e10
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/20870/
• CommitID: dbe7485

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/20904/
• CommitID: 0e8e1f9

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

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
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-8807-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 b9b5e5c676150f1148c2eb9306e3ce1c8a57c87c
# Push it to GitHub
git push --set-upstream origin backport/backport-8807-to-2.x
# Go back to the original working tree
popd
# 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-8807-to-2.x.