public Set<String> getClusters(Service service) {
        return serviceClusterIndex.getOrDefault(service, new HashSet<>());
    }
@Test
    void testGetClusters() {
        Set<String> clusters = serviceStorage.getClusters(SERVICE);
        
        assertNotNull(clusters);
        for (String cluster : clusters) {
            assertEquals(NACOS, cluster);
        }
    }