Add unit tests for RemoteReaderClient

[AndreKurait]

Description

Add unit tests. Followup from https://github.com/opensearch-project/opensearch-migrations/pull/1114/files

• Category: Test
• Why these changes are required? Explicitly test new code paths
• What is the old behavior before changes and new behavior after changes? No behavior change

Issues Resolved

N/A

Is this a backport? If so, please add backport PR # and/or commits #

Testing

Unit test

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• 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 ✅

Project coverage is 80.46%. Comparing base (e78d172) to head (616ebed).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1116      +/-   ##
============================================
+ Coverage     80.40%   80.46%   +0.05%     
- Complexity     2944     2947       +3     
============================================
  Files           393      393              
  Lines         14536    14536              
  Branches       1001     1001              
============================================
+ Hits          11688    11696       +8     
+ Misses         2242     2235       -7     
+ Partials        606      605       -1     

Flag	Coverage Δ
gradle-test	78.53% <ø> (+0.06%)	⬆️
python-test	90.33% <ø> (ø)

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[peternied]

Thanks