https://github.com/elastic/elasticsearch/pull/97304 removes several places that might do an expensive rebuild of the cluster state via `getStateForMasterService`. There's one other call we should be able to remove, in `JoinValidationService`, but it's a little tricky since we rely on seeing the last-accepted state for the cluster UUID validation, and it's not obviously ok to expose the last-accepted state without the no-master block.

This is a fairly minor issue because we do not do this join validation during elections, only if we're the elected master, and in that case `getStateForMasterService` is cheap. But that's not guaranteed, and we should strengthen this to avoid the that expensive step whenever possible.
server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java
}, this.singleNodeClusterSeedHostsCheckInterval, Names.SAME);
            }
            return;
        }
        
        if (singleNodeClusterChecker != null) {
            singleNodeClusterChecker.cancel();
            singleNodeClusterChecker = null;
        }
    }
    private void checkSingleNodeCluster() {
        if (mode != Mode.LEADER || applierState.nodes().size() > 1) {
            return;
        }
        if (DISCOVERY_SEED_HOSTS_SETTING.exists(settings)) {
            if (DISCOVERY_SEED_HOSTS_SETTING.get(settings).isEmpty()) {
                
                return;
            } else {
                logger.warn(
                    """
                        This node is a fully-formed single-node cluster with cluster UUID [{}], but it is configured as if to \
                        discover other nodes and form a multi-node cluster via the [{}] setting. Fully-formed clusters do not \
                        attempt to discover other nodes, and nodes with different cluster UUIDs cannot belong to the same cluster. \
                        The cluster UUID persists across restarts and can only be changed by deleting the contents of the node's \
                        data path(s). Remove the discovery configuration to suppress this message.""",
                    applierState.metadata().clusterUUID(),
                    DISCOVERY_SEED_HOSTS_SETTING.getKey() + "=" + DISCOVERY_SEED_HOSTS_SETTING.get(settings)
                );
            }