Function parse_param_value 

pub fn parse_param_value(
    decls: &IndexMap<String, ParamDecl>,
    name: &str,
    raw: &str,
) -> Result<ScalarValue, String>

Parse a caller-supplied parameter assignment (CLI --param k=v, server query string) against the document’s declarations. Unknown names, type mismatches, and out-of-range numbers are errors — hosts report them up front instead of silently clamping at eval time.