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 80.54%. Comparing base (29fafd6) to head (5033a31).

Additional details and impacted files

@@                           Coverage Diff                            @@
##             feature/rrf-score-normalization-v2     #997      +/-   ##
========================================================================
+ Coverage                                 79.80%   80.54%   +0.73%     
- Complexity                                 1027     1040      +13     
========================================================================
  Files                                        83       83              
  Lines                                      3536     3536              
  Branches                                    592      592              
========================================================================
+ Hits                                       2822     2848      +26     
+ Misses                                      465      434      -31     
- Partials                                    249      254       +5     

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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