### 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/TransportStartDataFrameAnalyticsAction.java
);
                }
                listener.onFailure(e);
            }
        };
        
        ActionListener<StartContext> memoryUsageHandledListener = ActionListener.wrap(startContext -> {
            TaskParams taskParams = new TaskParams(
                request.getId(),
                startContext.config.getVersion(),
                startContext.config.isAllowLazyStart()
            );
            persistentTasksService.sendStartRequest(
                MlTasks.dataFrameAnalyticsTaskId(request.getId()),
                MlTasks.DATA_FRAME_ANALYTICS_TASK_NAME,
                taskParams,
                waitForAnalyticsToStart
            );
        }, listener::onFailure);
        
        ActionListener<StartContext> startContextListener = ActionListener.wrap(
            startContext -> estimateMemoryUsageAndUpdateMemoryTracker(startContext, memoryUsageHandledListener),
            listener::onFailure
        );
        
        getStartContext(request.getId(), task, startContextListener);
    }
    private void estimateMemoryUsageAndUpdateMemoryTracker(StartContext startContext, ActionListener<StartContext> listener) {
        final String jobId = startContext.config.getId();