Add test coverage for ppc_data, ppd_data, ppc_ribbon_data, ppd_ribbon

[utkarshpawade]

Fixes #505

Summary

• Adds 8 tests for ppc_data() and ppd_data() in test-ppc-distributions.R covering structure validation, y/yrep row correctness, group column handling, and single-replicate edge case.
• Adds 2 tests for ppc_ribbon_data() and ppd_ribbon_data() in test-ppc-intervals.R confirming they are identical aliases to ppc_intervals_data() / ppd_intervals_data().

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (b19e3cd) to head (39555c1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #506   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          35       35           
  Lines        5904     5904           
=======================================
  Hits         5826     5826           
  Misses         78       78           

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