Function run_nested 

pub fn run_nested(
    ctx: &mut OpCtx<'_>,
    name: &str,
    pairs: &[(&str, &str)],
    inputs: Vec<PathBuf>,
    output: Option<&Path>,
) -> Result<(Value, Vec<OutputRecord>)>

Run any registered op as a nested step, with its own inputs and output path. Returns the op’s result value and the outputs it wrote. Warnings and models are merged.