Function insert_line_before 

pub fn insert_line_before(text: &str, idx: usize, line: &str) -> String

Insert line before line index idx, preserving the file’s line endings and its trailing-newline habit.

line may itself be several lines (a generated method body); each is re-emitted in the target file’s style, so a multi-line insert into a CRLF file doesn’t leave bare \n behind.

Prefer declare_module or insert_before_marker — they find their own anchor and decline when the file isn’t what they expected. Reach for this only when the caller has located the line itself.