### 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
String resultsMappingAfterJob1 = EntityUtils.toString(client().performRequest(getResultsMappingRequest).getEntity());
        assertThat(resultsMappingAfterJob1, containsString(byFieldName1));
        assertThat(resultsMappingAfterJob1, not(containsString(byFieldName2)));
        putJob(jobId2, String.format(Locale.ROOT, jobTemplate, byFieldName2));
        
        String resultsMappingAfterJob2 = EntityUtils.toString(client().performRequest(getResultsMappingRequest).getEntity());
        assertThat(resultsMappingAfterJob2, containsString(byFieldName1));
        assertThat(resultsMappingAfterJob2, containsString(byFieldName2));
    }
    public void testCreateJob_WithClashingFieldMappingsFails() throws Exception {
        String jobTemplate = """
            {
              "analysis_config" : {
                    "detectors" :[{"function":"metric","field_name":"metric", "by_field_name":"%s"}]
                },
              "data_description": {}
            }""";
        String jobId1 = "job-with-response-field";
        String byFieldName1;
        String jobId2 = "job-will-fail-with-mapping-error-on-response-field";
        String byFieldName2;
        
        if (randomBoolean()) {
            byFieldName1 = "response";
            byFieldName2 = "response.time";
        } else {
            byFieldName1 = "response.time";
            byFieldName2 = "response";
        }
        putJob(jobId1, String.format(Locale.ROOT, jobTemplate, byFieldName1));