pub fn pkce_pair() -> (String, String)Expand description
Generate a random PKCE verifier and matching S256 code challenge (RFC 7636 §4.1, §4.2).
The verifier is 32 bytes of CSPRNG output encoded as base64url-no-pad,
landing at 43 ASCII characters — within the RFC’s 43..=128 range.
The challenge is BASE64URL-NO-PAD(SHA256(verifier)).