Function generate_pkce_challenge 

pub fn generate_pkce_challenge() -> Result<PkceChallenge>

Generate a cryptographically secure PKCE challenge pair.

This function generates a random code verifier and computes the corresponding S256 code challenge.

Example

use vtcode_auth::generate_pkce_challenge;

let challenge = generate_pkce_challenge().unwrap();
// code_verifier is a secret — never print or log it.
assert!(!challenge.code_challenge.is_empty());