### 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
}
                } catch (IndexNotFoundException | ShardNotFoundException e) {
                    
                    
                    
                    if (allowPartialSearchResults == false) {
                        throw e;
                    }
                }
                OriginalIndices finalIndices = new OriginalIndices(
                    new String[] { shardId.getIndexName() },
                    originalIndices.indicesOptions()
                );
                iterators.add(
                    new SearchShardIterator(
                        localClusterAlias,
                        shardId,
                        targetNodes,
                        finalIndices,
                        perNode.getSearchContextId(),
                        keepAlive,
                        false,
                        false
                    )
                );
            }
        }
        return iterators;
    }
    List<SearchShardIterator> getLocalShardsIterator(