Fix tests error.

Author: afoucret

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/services/elastic/request/ElasticInferenceServiceUnifiedChatCompletionRequestTests.java

@@ -50,7 +50,7 @@ public void testCreateHttpRequest_SingleInput() throws IOException {
         assertThat(httpPost.getLastHeader(HttpHeaders.CONTENT_TYPE).getValue(), is(XContentType.JSON.mediaType()));
 
         var requestMap = entityAsMap(httpPost.getEntity().getContent());
-        assertThat(requestMap, aMapWithSize(3));
+        assertThat(requestMap, aMapWithSize(4));
         assertThat(requestMap.get("model"), is(modelId));
         assertThat(requestMap.get("n"), is(1));
         assertThat(requestMap.get("stream"), is(false));