Replace sapply() with vapply() for type safety

[utkarshpawade]

Description

Nine uses of sapply() can silently return a list() instead of the expected atomic vector on zero-length input, causing hard-to-diagnose downstream failures. vapply() enforces a declared return type and is faster due to pre-allocation.

The two sapply(x, colnames) calls at helpers-mcmc.R:277,303 should be intentionally kept as they rely on sapply()'s flexible return to detect mismatched column names across chains.