### 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/TransportSearchAction.java
return false;
        }
        if (searchRequest.searchType() == DFS_QUERY_THEN_FETCH) {
            return false;
        }
        if (searchRequest.hasKnnSearch()) {
            return false;
        }
        SearchSourceBuilder source = searchRequest.source();
        return source == null
            || source.collapse() == null
            || source.collapse().getInnerHits() == null
            || source.collapse().getInnerHits().isEmpty();
    }
    /**
     * Handles ccs_minimize_roundtrips=true
     */
    static void ccsRemoteReduce(
        TaskId parentTaskId,
        SearchRequest searchRequest,
        OriginalIndices localIndices,
        Map<String, OriginalIndices> remoteIndices,
        SearchResponse.Clusters clusters,
        SearchTimeProvider timeProvider,
        AggregationReduceContext.Builder aggReduceContextBuilder,
        RemoteClusterService remoteClusterService,
        ThreadPool threadPool,
        ActionListener<SearchResponse> listener,
        BiConsumer<SearchRequest, ActionListener<SearchResponse>> localSearchConsumer
    ) {