### Description

This applies to the following APIs:
- [x] open anomaly detection
- [x] start datafeed
- [x] start data frame analytics job
- [x] start trained model deployment

Currently, if the operation takes longer than the configured `timeout`, the response status code is `500`.

With Kibana in mind, a `4xx` response would be more suitable as it will lead to showing more useful information to the user about why the request failed.

Looking at [RFC 9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-408-request-timeout), it seems the most appropriate `4xx` code would be `408 Request Timeout`, even though the RFC describes a different situation. 

The alternative, `504 Gateway Timeout`, would be more appropriate for this situation but it wouldn't improve the user experience in Kibana.
x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/action/TransportStartDataFrameAnalyticsAction.java
licenseState
            );
        }
        @Override
        public PersistentTasksCustomMetadata.Assignment getAssignment(
            TaskParams params,
            Collection<DiscoveryNode> candidateNodes,
            @SuppressWarnings("HiddenField") ClusterState clusterState
        ) {
            boolean isMemoryTrackerRecentlyRefreshed = memoryTracker.isRecentlyRefreshed();
            Optional<PersistentTasksCustomMetadata.Assignment> optionalAssignment = getPotentialAssignment(
                params,
                clusterState,
                isMemoryTrackerRecentlyRefreshed
            );
            
            if (optionalAssignment.isPresent()) {
                return optionalAssignment.get();
            }
            JobNodeSelector jobNodeSelector = new JobNodeSelector(
                clusterState,
                candidateNodes,
                params.getId(),
                MlTasks.DATA_FRAME_ANALYTICS_TASK_NAME,
                memoryTracker,
                params.isAllowLazyStart() ? Integer.MAX_VALUE : maxLazyMLNodes,
                node -> nodeFilter(node, params)
            );