### 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/TransportOpenJobAction.java
@Override
                    public void onResponse(PersistentTasksCustomMetadata.PersistentTask<OpenJobAction.JobParams> task) {
                        waitForJobStarted(task.getId(), jobParams, clearJobFinishTime);
                    }
                    @Override
                    public void onFailure(Exception e) {
                        if (ExceptionsHelper.unwrapCause(e) instanceof ResourceAlreadyExistsException) {
                            e = new ElasticsearchStatusException(
                                "Cannot open job [{}] because it has already been opened",
                                RestStatus.CONFLICT,
                                e,
                                jobParams.getJobId()
                            );
                        }
                        listener.onFailure(e);
                    }
                };
            
            ActionListener<Long> memoryRequirementRefreshListener = ActionListener.wrap(
                mem -> persistentTasksService.sendStartRequest(
                    MlTasks.jobTaskId(jobParams.getJobId()),
                    MlTasks.JOB_TASK_NAME,
                    jobParams,
                    waitForJobToStart
                ),
                listener::onFailure
            );
            
            ActionListener<Boolean> modelSnapshotValidationListener = ActionListener.wrap(
                response -> memoryTracker.refreshAnomalyDetectorJobMemoryAndAllOthers(