Function normalize_engine_arguments 

pub fn normalize_engine_arguments(
    signature: &Signature,
    args: Vec<Argument>,
) -> Result<Vec<Argument>, ShellError>

Apply signature-aware null handling and expand record spreads for engine Arguments.

List spreads are rejected when the command has no rest parameter (and does not allow unknown args), so ...$list cannot be silently dropped on named-only commands.