Add integration and unit tests for missing RRF coverage

[ryanbogan]

Description

This PR adds a successful end to end integration test using RRF, as well as unit tests for the RRFProcessor class, which was the last remaining RRF class without tests.

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[peterzhuamazon]

Please rebase to see the codecov comments.

Thanks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.59%. Comparing base (29fafd6) to head (71b1d76).
Report is 1 commits behind head on feature/rrf-score-normalization-v2.

Additional details and impacted files

@@                           Coverage Diff                            @@
##             feature/rrf-score-normalization-v2     #997      +/-   ##
========================================================================
+ Coverage                                 79.80%   80.59%   +0.79%     
- Complexity                                 1027     1043      +16     
========================================================================
  Files                                        83       83              
  Lines                                      3536     3536              
  Branches                                    592      592              
========================================================================
+ Hits                                       2822     2850      +28     
+ Misses                                      465      434      -31     
- Partials                                    249      252       +3     

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[martin-gaievski]

@ryanbogan I've rebased on latest main out of curiosity, code coverage report/bot works. Please make sure to pull latest changes before making any edits locally.

[martin-gaievski]

Code looks good to me. Just one ask - can you please add the BWC test, similar to what is there for normalization
https://github.com/opensearch-project/neural-search/blob/main/qa/restart-upgrade/src/test/java/org/opensearch/neuralsearch/bwc/HybridSearchIT.java
and
https://github.com/opensearch-project/neural-search/blob/main/qa/rolling-upgrade/src/test/java/org/opensearch/neuralsearch/bwc/HybridSearchIT.java

[martin-gaievski]

Code looks good to me. Just one ask - can you please add the BWC test, similar to what is there for normalization https://github.com/opensearch-project/neural-search/blob/main/qa/restart-upgrade/src/test/java/org/opensearch/neuralsearch/bwc/HybridSearchIT.java and https://github.com/opensearch-project/neural-search/blob/main/qa/rolling-upgrade/src/test/java/org/opensearch/neuralsearch/bwc/HybridSearchIT.java

As discussed offline, let's hold off on BWC tests, we can do it after RRF is merged to main from feature branch, otherwise it will keep failing for 2.19 to main/3.0