@Override
            public void handle(Callback[] callbacks) throws UnsupportedCallbackException {
                for (Callback callback : callbacks) {
                    if (callback instanceof OAuthBearerValidatorCallback) {
                        OAuthBearerValidatorCallback validationCallback = (OAuthBearerValidatorCallback) callback;
                        validationCallback.token(new OAuthBearerTokenMock());
                    } else if (callback instanceof OAuthBearerExtensionsValidatorCallback) {
                        OAuthBearerExtensionsValidatorCallback extensionsCallback = (OAuthBearerExtensionsValidatorCallback) callback;
                        extensionsCallback.error("firstKey", "is not valid");
                        extensionsCallback.error("secondKey", "is not valid either");
                    } else
                        throw new UnsupportedCallbackException(callback);
                }
            }
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