Extract drop_singleton_values() helper in mcmc_nuts_treedepth()

[utkarshpawade]

Fixes #511

Summary

• Extract drop_singleton_values(df, col) internal helper to deduplicate the "keep values with count > 1" filtering logic
• Replace both instances in mcmc_nuts_treedepth() with calls to the helper

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (b19e3cd) to head (1f53b21).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
- Coverage   98.67%   98.67%   -0.01%     
==========================================
  Files          35       35              
  Lines        5904     5903       -1     
==========================================
- Hits         5826     5825       -1     
  Misses         78       78              

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[jgabry]

Thanks!