Function check_process_success 

pub fn check_process_success(
    output: &Output,
    intent: &'static str,
) -> Result<(), Error>

Ensure that the tmux command succeeded (exit status 0) before parsing its output.

This prevents confusing parse errors when tmux fails and returns empty or garbage stdout. Instead, we get a clear error with the actual stderr message.

Errors

Returns Error::UnexpectedTmuxOutput if the command exited with non-zero status.