### 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
PersistentTasksCustomMetadata.Assignment assignment = jobNodeSelector.selectNode(
                maxOpenJobs,
                Integer.MAX_VALUE,
                maxMachineMemoryPercent,
                maxNodeMemory,
                useAutoMemoryPercentage
            );
            auditRequireMemoryIfNecessary(params.getId(), auditor, assignment, jobNodeSelector, isMemoryTrackerRecentlyRefreshed);
            return assignment;
        }
        @Override
        protected void nodeOperation(AllocatedPersistentTask task, TaskParams params, PersistentTaskState state) {
            DataFrameAnalyticsTask dfaTask = (DataFrameAnalyticsTask) task;
            DataFrameAnalyticsTaskState analyticsTaskState = (DataFrameAnalyticsTaskState) state;
            DataFrameAnalyticsState analyticsState = analyticsTaskState == null
                ? DataFrameAnalyticsState.STOPPED
                : analyticsTaskState.getState();
            logger.info("[{}] Starting data frame analytics from state [{}]", params.getId(), analyticsState);
            
            if (DataFrameAnalyticsState.STOPPING.equals(analyticsState)) {
                logger.info("[{}] data frame analytics got reassigned while stopping. Marking as completed", params.getId());
                task.markAsCompleted();
                return;
            }
            
            if (DataFrameAnalyticsState.FAILED.equals(analyticsState)) {
                return;
            }
            
            ActionListener<GetDataFrameAnalyticsStatsAction.Response> statsListener = ActionListener.wrap(statsResponse -> {