Add unit tests for tidy parameter selection helpers

[utkarshpawade]

param_range(), param_glue(), and tidyselect_parameters() in R/tidy-params.R currently have no dedicated unit tests. The only coverage comes indirectly through prepare_mcmc_array() integration tests in test-helpers-mcmc.R.

This leaves gaps in:

• Standalone behavior (correct index selection, partial matches)
• Edge cases (no matches, single index)
• Error handling (vars type validation)

A dedicated tests/testthat/test-tidy-params.R file should be added.