Function parse_nl_text_with_quadratic 

pub fn parse_nl_text_with_quadratic(
    txt: &str,
    use_quadratic: bool,
) -> Result<NlProblem, String>

parse_nl_text with parse-time quadratic recognition explicitly on or off.

The env var the plain entry point reads is process-global, which is exactly wrong for a differential test that has to parse the same text both ways in one process and compare the results — so the knob is a parameter here, mirroring NlTnlp::try_new_with_quadratic.