### 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/TransportStartDatafeedAction.java
if (DatafeedState.STOPPING.equals(datafeedState)) {
                logger.info("[{}] datafeed got reassigned while stopping. Marking as completed", params.getDatafeedId());
                datafeedTask.completeOrFailIfRequired(null);
                return;
            }
            switch (datafeedTask.setDatafeedRunner(datafeedRunner)) {
                case NEITHER -> datafeedRunner.run(datafeedTask, datafeedTask::completeOrFailIfRequired);
                case ISOLATED -> logger.info("[{}] datafeed isolated immediately after reassignment.", params.getDatafeedId());
                case STOPPED -> {
                    logger.info("[{}] datafeed stopped immediately after reassignment. Marking as completed", params.getDatafeedId());
                    datafeedTask.completeOrFailIfRequired(null);
                }
            }
        }
        @Override
        protected AllocatedPersistentTask createTask(
            long id,
            String type,
            String action,
            TaskId parentTaskId,
            PersistentTasksCustomMetadata.PersistentTask<StartDatafeedAction.DatafeedParams> persistentTask,
            Map<String, String> headers
        ) {
            return new DatafeedTask(id, type, action, parentTaskId, persistentTask.getParams(), headers);
        }
    }
    public static class DatafeedTask extends AllocatedPersistentTask implements StartDatafeedAction.DatafeedTaskMatcher {