Function parse_gen_dir 

pub(crate) fn parse_gen_dir(args: &[String], usage: &str) -> Result<PathBuf>

Parses the single positional <gen-dir> argument shared by the codex-schema regen and codex-schema bisect subcommands: --help/-h prints usage and exits 0 immediately, exactly one positional argument is accepted as the generated-schema directory, and any further positional argument is rejected. usage doubles as the error context when the positional argument is missing.