Add test coverage for mcmc_areas_ridges_data, mcmc_parcoord_data, and mcmc_trace_data

[utkarshpawade]

Summary

The exported *_data() functions mcmc_areas_ridges_data(), mcmc_parcoord_data(), and mcmc_trace_data() currently lack dedicated unit tests. While their corresponding plotting functions have visual regression tests via vdiffr, the data-preparation functions themselves are not directly tested for correct output structure, column names, argument handling, or error conditions.

• mcmc_areas_ridges_data() : No tests for output structure (parameter, x, density, interval columns), delegation to mcmc_areas_data() with point_est = "none", or multi-parameter usage.
• mcmc_parcoord_data() : No tests for output structure (Draw, Parameter, Value, Divergent columns), default Divergent = 0 when np = NULL, the np argument path, the minimum 2-parameter requirement, or regex_pars support.
• mcmc_trace_data() : No tests for output structure and dimensions, highlight, n_warmup, iter1, negative iter1 error, mutual exclusivity of n_warmup/iter1, multi-parameter support, or value_rank computation.