### 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
"Cannot open jobs because persistent task assignment is disabled by the "
                        + "[cluster.persistent_tasks.allocation.enable] setting"
                )
            );
        } finally {
            
            
            Request enablePersistentTaskAssignmentRequest = new Request("PUT", "_cluster/settings");
            enablePersistentTaskAssignmentRequest.setJsonEntity("""
                {
                  "persistent": {
                    "cluster.persistent_tasks.allocation.enable": "all"
                  }
                }""");
            Response enablePersistentTaskAssignmentResponse = client().performRequest(disablePersistentTaskAssignmentRequest);
            assertThat(entityAsMap(enablePersistentTaskAssignmentResponse), hasEntry("acknowledged", true));
        }
    }
    public void testDeleteJob() throws Exception {
        String jobId = "delete-job-job";
        String indexName = AnomalyDetectorsIndexFields.RESULTS_INDEX_PREFIX + AnomalyDetectorsIndexFields.RESULTS_INDEX_DEFAULT;
        createFarequoteJob(jobId);
        
        String indicesBeforeDelete = EntityUtils.toString(
            client().performRequest(new Request("GET", "/_cat/indices/" + AnomalyDetectorsIndexFields.RESULTS_INDEX_PREFIX + "*"))
                .getEntity()
        );
        assertThat(indicesBeforeDelete, containsString(indexName));
        client().performRequest(new Request("DELETE", MachineLearning.BASE_PATH + "anomaly_detectors/" + jobId));