Function extract_page_function 

pub fn extract_page_function<'js>(
    ctx: &Ctx<'js>,
    value: Value<'js>,
) -> Result<(String, Option<bool>)>

Extract (fn_source, is_function_hint) from an evaluate pageFunction arg that can be a JS string or a JS function — matches Playwright’s String(pageFunction) + typeof pageFunction === 'function' check. For functions, invokes the engine’s Function.prototype.toString() via global String(...).