public ShenYuTestCaseSpecBuilder addNotExists(final String endpoint, final String message) {
            return add(WebSocketCheckers.notExists(endpoint, message));
        }
add not exists case spec.
@param endpoint endpoint
@param message message
@return ShenYuTestCaseSpecBuilder
shenyu-e2e/shenyu-e2e-engine/src/main/java/org/apache/shenyu/e2e/engine/scenario/specification/ShenYuCaseSpec.java