Function collect_missing_params 

pub fn collect_missing_params(inputs: &Punctuated<FnArg, Comma>) -> Vec<String>

Collect parameter names that have Missing<T> types.

These parameters need an automatic R default of quote(expr=) which evaluates to R_MissingArg (the “missing argument” sentinel). Names are normalized via normalize_r_arg_ident.

Returns normalized parameter names in declaration order.