### 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
MIN_REPORTED_SUPPORTED_SNAPSHOT_VERSION.toString()
                            )
                        );
                    }, failure -> {
                        if (ExceptionsHelper.unwrapCause(failure) instanceof ResourceNotFoundException) {
                            modelSnapshotValidationListener.onResponse(true);
                            return;
                        }
                        listener.onFailure(ExceptionsHelper.serverError("Unable to validate model snapshot", failure));
                    })
                );
            }, listener::onFailure);
            
            jobConfigProvider.getJob(jobParams.getJobId(), null, ActionListener.wrap(builder -> {
                jobParams.setJob(builder.build());
                getJobHandler.onResponse(null);
            }, listener::onFailure));
        } else {
            listener.onFailure(LicenseUtils.newComplianceException(XPackField.MACHINE_LEARNING));
        }
    }
    private void waitForJobStarted(String taskId, OpenJobAction.JobParams jobParams, ActionListener<NodeAcknowledgedResponse> listener) {
        JobPredicate predicate = new JobPredicate();
        persistentTasksService.waitForPersistentTaskCondition(
            taskId,
            predicate,
            jobParams.getTimeout(),
            new PersistentTasksService.WaitForPersistentTaskListener<OpenJobAction.JobParams>() {
                @Override
                public void onResponse(PersistentTasksCustomMetadata.PersistentTask<OpenJobAction.JobParams> persistentTask) {