nuts_params.CmdStanMCMC() drops array dimension when selecting a single parameter

[utkarshpawade]

In R/bayesplot-extractors.R:178, the subsetting arr[,, pars] drops the third dimension when pars has length 1, collapsing the 3D array into a 2D matrix. This breaks reshape2::melt() and downstream code that expects a 3D array.