### 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/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/DatafeedJobsIT.java
if (getDataCounts(job.getId()).getProcessedRecordCount() < numDocs) {
                assertThat(getModelSnapshots(jobId), hasSize(0));
            }
        } else {
            assertThat(getModelSnapshots(jobId), hasSize(1));
        }
    }
    public void testRealtime_noDataAndAutoStop() throws Exception {
        String jobId = "realtime-job-auto-stop";
        String datafeedId = jobId + "-datafeed";
        startRealtime(jobId, randomIntBetween(1, 3));
        
        assertBusy(() -> {
            GetDatafeedsStatsAction.Request request = new GetDatafeedsStatsAction.Request(datafeedId);
            GetDatafeedsStatsAction.Response response = client().execute(GetDatafeedsStatsAction.INSTANCE, request).actionGet();
            assertThat(response.getResponse().results().get(0).getDatafeedState(), equalTo(DatafeedState.STOPPED));
        });
        
        assertBusy(() -> {
            GetJobsStatsAction.Request request = new GetJobsStatsAction.Request(jobId);
            GetJobsStatsAction.Response response = client().execute(GetJobsStatsAction.INSTANCE, request).actionGet();
            assertThat(response.getResponse().results().get(0).getState(), equalTo(JobState.CLOSED));
        });
    }
    public void testRealtime_multipleStopCalls() throws Exception {
        String jobId = "realtime-job-multiple-stop";