Add test coverage for mcmc_areas_ridges_data, mcmc_parcoord_data, mcmc_trace_data

[utkarshpawade]

Fixes #507

Summary

• Add unit tests for mcmc_areas_ridges_data() (4 tests): output structure, delegation to mcmc_areas_data(), multi-parameter and single-parameter cases
• Add unit tests for mcmc_parcoord_data() (5 tests): output structure, default divergent column, np argument, minimum 2-parameter error, regex_pars support
• Add unit tests for mcmc_trace_data() (9 tests): output structure/dimensions, highlight, n_warmup, iter1, error conditions, multi-parameter support, value_rank correctness

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (b19e3cd) to head (90c8d5b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #508   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          35       35           
  Lines        5904     5904           
=======================================
  Hits         5826     5826           
  Misses         78       78           

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