Add unit tests for tidy parameter selection helpers

[utkarshpawade]

Fixes #492

Summary

• Add tests/testthat/test-tidy-params.R with 19 unit tests covering param_range(), param_glue(), and tidyselect_parameters()
• Tests cover correct selection, edge cases (no matches, partial matches, single index), error handling, and integration with vars() via prepare_mcmc_array()

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (a30a706) to head (ab18e91).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #493   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files          35       35           
  Lines        5857     5857           
=======================================
  Hits         5779     5779           
  Misses         78       78           

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