shorten NA warning and add vdiffr tests for NA draws

Author: utkarshpawade

R/helpers-mcmc.R

@@ -30,7 +30,7 @@ prepare_mcmc_array <- function(x,
   }
   if (anyNA(x)) {
     warn(
-      "NAs were found in 'x'. `prepare_mcmc_array()` does not remove them; some plots may render with missing values dropped, while summary functions (e.g. intervals, densities, diagnostics) may produce misleading results or error. Consider removing NAs before plotting or summarizing."
+      "NAs found in 'x'. These are passed through as-is and may affect the resulting plots."
     )
   }