Replace sapply() with vapply() for type safety

[utkarshpawade]

Fixes #516

Summary

• Replaces 9 uses of sapply() with vapply() across 5 files
• vapply() enforces a declared return type, preventing silent list() returns on zero-length input that can cause hard to diagnose downstream failures
• Two sapply(x, colnames) calls at helpers-mcmc.R:277,303 are intentionally unchanged, they rely on flexible return types to detect mismatched column names across chains

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (94a2ac5) to head (0bb70dd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #517   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          35       35           
  Lines        5903     5903           
=======================================
  Hits         5825     5825           
  Misses         78       78           

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