pub fn validate_code_verifier(code_verifier: &str) -> Result<(), OAuthError>Expand description
Validates a PKCE code_verifier against RFC 7636 §4.1 syntax: 43–128
characters drawn only from the unreserved set A-Z, a-z, 0-9, -,
., _, ~. Empty, too-short, too-long, whitespace, non-ASCII, and
reserved-character values are rejected so RustAuth never advertises S256
for a verifier a provider would reject.