### Description

`SearchResponse.Clusters` currently uses a `HashMap<String, AtomicReference>` for storing Cluster objects. The AtomicReference CAS swaps are done in a do/while loop in most cases (e.g., in `CCSSingleCoordinatorSearchProgressListener`).

This can be replaced with a `ConcurrentHashMap` in conjunction with either `compute()` method or the `replace(key, assumed_previous_value, value)` method for atomically swapping in a new Cluster object with a CAS operation, assuring that the updates don't lose data from competing threads doing separate Cluster object updates.

See: https://github.com/elastic/elasticsearch/pull/98457/files#r1310467586 
server/src/main/java/org/elasticsearch/action/search/CCSSingleCoordinatorSearchProgressListener.java
.build();
                swapped = clusterRef.compareAndSet(curr, updated);
            } while (swapped == false);
        }
    }
    /**
     * Executed when a shard returns a fetch result.
     *
     * @param shardIndex The index of the shard in the list provided by {@link SearchProgressListener#onListShards})}.
     */
    @Override
    public void onFetchResult(int shardIndex) {}
    /**
     * Executed when a shard reports a fetch failure.
     *
     * @param shardIndex The index of the shard in the list provided by {@link SearchProgressListener#onListShards})}.
     * @param shardTarget The last shard target that thrown an exception.
     * @param exc The cause of the failure.
     */
    @Override
    public void onFetchFailure(int shardIndex, SearchShardTarget shardTarget, Exception exc) {}
    private Map<String, Integer> partitionCountsByClusterAlias(List<SearchShard> shards) {
        final Map<String, Integer> res = new HashMap<>();
        for (SearchShard shard : shards) {
            res.merge(Objects.requireNonNullElse(shard.clusterAlias(), RemoteClusterAware.LOCAL_CLUSTER_GROUP_KEY), 1, Integer::sum);
        }
        return res;
    }
}