### 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
public void onResponse(PersistentTasksCustomMetadata.PersistentTask<?> task) {
                
                
                listener.onFailure(exception);
            }
            @Override
            public void onFailure(Exception e) {
                logger.error(
                    () -> format(
                        "[%s] Failed to cancel persistent task that could not be assigned due to [%s]",
                        persistentTask.getParams().getJobId(),
                        exception.getMessage()
                    ),
                    e
                );
                listener.onFailure(exception);
            }
        });
    }
    /**
     * This class contains the wait logic for waiting for a job's persistent task to be allocated on
     * job opening.  It should only be used in the open job action, and never at other times the job's
     * persistent task may be assigned to a node, for example on recovery from node failures.
     *
     * Important: the methods of this class must NOT throw exceptions.  If they did then the callers
     * of endpoints waiting for a condition tested by this predicate would never get a response.
     */
    private static class JobPredicate implements Predicate<PersistentTasksCustomMetadata.PersistentTask<?>> {
        private volatile Exception exception;
        private volatile String node = "";