### 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/MlJobIT.java
}
        }).build());
        Response response = client().performRequest(aliasesRequest);
        return EntityUtils.toString(response.getEntity());
    }
    private void openJob(String jobId) throws IOException {
        Response openResponse = client().performRequest(
            new Request("POST", MachineLearning.BASE_PATH + "anomaly_detectors/" + jobId + "/_open")
        );
        assertThat(entityAsMap(openResponse), hasEntry("opened", true));
    }
    private void closeJob(String jobId) throws IOException {
        Response openResponse = client().performRequest(
            new Request("POST", MachineLearning.BASE_PATH + "anomaly_detectors/" + jobId + "/_close")
        );
        assertThat(entityAsMap(openResponse), hasEntry("closed", true));
    }
    private Response putJob(String jobId, String jsonBody) throws IOException {
        Request request = new Request("PUT", MachineLearning.BASE_PATH + "anomaly_detectors/" + jobId);
        request.setJsonEntity(jsonBody);
        return client().performRequest(request);
    }
    @After
    public void clearMlState() throws Exception {
        new MlRestTestStateCleaner(logger, adminClient()).resetFeatures();
        
        waitForPendingTasks(adminClient(), taskName -> taskName.contains(MlTasks.DATA_FRAME_ANALYTICS_TASK_NAME));
    }
}