Increase AbstractRetrieverToolTests code coverage

[mingshl]

Description

increase AbstractRetrieverToolTests code coverage

Issues Resolved

mentioned in #40

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

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (db49d34) 44.41% compared to head (1df9138) 46.08%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #53      +/-   ##
============================================
+ Coverage     44.41%   46.08%   +1.67%     
- Complexity       30       31       +1     
============================================
  Files             6        6              
  Lines           358      358              
  Branches         42       42              
============================================
+ Hits            159      165       +6     
+ Misses          178      172       -6     
  Partials         21       21              

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[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:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/skills/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/skills/backport-2.x
# Create a new branch
git switch --create backport-53-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 917aed5222dda4245e9077b731a3ec470622fcb4
# Push it to GitHub
git push --set-upstream origin backport-53-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/skills/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport-53-to-2.x.

[zhichao-aws]

The automatic backport failed. Could you please backport this PR manually?