SOLR-15569: Split on the right subtree when node value == threshold on MultipleAdditiveTreesModel

[spyk]

https://issues.apache.org/jira/browse/SOLR-15569

Description

Fixes an issue where the MultipleAdditiveTreesModel does not split correctly when the tree node value equals the split threshold. This was discovered while testing a translated XGBoost model for LTR and getting slightly different score results.

NOTE: The previous logic split to the left and also added a NODE_SPLIT_SLACK that has been removed. Not sure if this original logic was part of another model, or served another purpose.

Solution

• Changed split logic to split on the right subtree when the node value equals the threshold
• Removed NODE_SPLIT_SLACK completely

Tests

• Added TestMultipleAdditiveTreesModel.testMultipleAdditiveTreesSplitAtThreshold test to showcase the issue

Checklist

Please review the following and check all that apply:

• [X] I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• [X] I have created a Jira issue and added the issue ID to my pull request title.
• [X] I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• [X] I have developed this patch against the main branch.
• [X] I have run ./gradlew check.
• [X] I have added tests for my changes.
• [ ] I have added documentation for the Reference Guide

[spyk]

I took a stab on the 'splitToRight' flag implementation. Please review and let me know of any issues, thanks!

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