### 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
}
            PersistentTasksCustomMetadata.Assignment assignment = persistentTask.getAssignment();
            if (assignment != null) {
                
                if (assignment.equals(DatafeedNodeSelector.AWAITING_JOB_ASSIGNMENT)) {
                    return true;
                }
                
                if (assignment.equals(DatafeedNodeSelector.AWAITING_JOB_RELOCATION)) {
                    return true;
                }
                if (assignment.equals(PersistentTasksCustomMetadata.INITIAL_ASSIGNMENT) == false && assignment.isAssigned() == false) {
                    
                    exception = new ElasticsearchStatusException(
                        "Could not start datafeed, allocation explanation [" + assignment.getExplanation() + "]",
                        RestStatus.TOO_MANY_REQUESTS
                    );
                    return true;
                }
            }
            DatafeedState datafeedState = (DatafeedState) persistentTask.getState();
            if (datafeedState == DatafeedState.STARTED) {
                node = persistentTask.getExecutorNode();
                return true;
            }
            return false;
        }
    }
}