Function stream_process_command_lines 

pub fn stream_process_command_lines(
    spec: &ProcessCommandSpec,
    stdin: Vec<u8>,
    on_line: impl FnMut(&[u8]) -> Result<()>,
) -> Result<Vec<u8>>

Runs the command in spec, feeding stdin, invoking on_line for each stdout line as it arrives, and returns the full captured stdout.