Function verify_response 

pub fn verify_response(
    challenge: &Challenge,
    resp: &ChallengeResponse,
) -> Result<(), String>

Verify that a response answers the given challenge and carries a valid signature.

Checks nonce agreement, decodes the embedded public key (32 bytes) and signature (64 bytes), then verifies the signature over the canonical message. Returns Ok(()) only when the response is cryptographically sound. A caller must still confirm which DID responded via responder_is.