Add dedicated edge-case tests for exported _data() functions

[utkarshpawade]

Fixes #520

Summary

• Adds test-data-functions-edge-cases.R covering edge cases not tested elsewhere
• Tests single observation, single draw, zero-length inputs, and alternative code paths for all exported _data() functions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (94a2ac5) to head (ed8b058).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          35       35           
  Lines        5903     5903           
=======================================
  Hits         5825     5825           
  Misses         78       78           

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