Function to_snake_case 

pub fn to_snake_case(value: &str) -> String

Converts value to snake_case by inserting a single _ before every non-initial uppercase character. Every other character — including any pre-existing _ — passes through unchanged.

This is intentionally not a natural-language word tokenizer: it does not treat _/-/ as separators to drop and rejoin. A model named User_Group therefore becomes user__group (double underscore), not user_group — that looks odd, but it is what the server’s route registration has always produced, and every client generator must match it exactly to avoid constructing a URL the server never registers.