Function issue_session_token 

pub fn issue_session_token(
    bundle: &ProofBundle,
    result: &VerifyResult,
    session_id: &str,
    issued_at: i64,
    valid_until: i64,
    session_secret: &[u8],
) -> Result<SessionToken, String>

Issue a SessionToken from a previously verified bundle’s result. Callers MUST only invoke this after verify_bundle returned valid=true.