private byte[] clientInitialResponse(String authorizationId, boolean illegalToken, Map<String, String> customExtensions)
            throws OAuthBearerConfigException, IOException, UnsupportedCallbackException {
        OAuthBearerTokenCallback callback = new OAuthBearerTokenCallback();
        LOGIN_CALLBACK_HANDLER.handle(new Callback[] {callback});
        OAuthBearerToken token = callback.token();
        String compactSerialization = token.value();

        String tokenValue = compactSerialization + (illegalToken ? "AB" : "");
        return new OAuthBearerClientInitialResponse(tokenValue, authorizationId, new SaslExtensions(customExtensions)).toBytes();
    }
If the callback handler handles the `OAuthBearerExtensionsValidatorCallback`
 and finds an invalid extension, SaslServer should throw an authentication exception
clients/src/test/java/org/apache/kafka/common/security/oauthbearer/internals/OAuthBearerSaslServerTest.java