Forward port changes from backport of #125562 (#126413)

The backport to `8.x` needed some changes to pass through CI; this
commit forward-ports the relevant bits of those changes back into `main`
to keep the branches aligned.

test/framework/src/main/java/org/elasticsearch/rest/RestResponseUtils.java

@@ -12,14 +12,19 @@ package org.elasticsearch.rest;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.core.CheckedConsumer;
-import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xcontent.ToXContent;
+import org.elasticsearch.xcontent.XContentBuilder;
 
 import java.io.IOException;
 import java.io.StringWriter;
 import java.io.Writer;
 import java.util.Iterator;
 
+import static org.elasticsearch.test.ESTestCase.asInstanceOf;
+import static org.elasticsearch.test.ESTestCase.fail;
 import static org.elasticsearch.transport.BytesRefRecycler.NON_RECYCLING_INSTANCE;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
 
 public class RestResponseUtils {
     private RestResponseUtils() {}
@@ -48,7 +53,7 @@ public class RestResponseUtils {
             out.flush();
             return out.bytes();
         } catch (Exception e) {
-            return ESTestCase.fail(e);
+            return fail(e);
         }
     }
 
@@ -60,7 +65,18 @@ public class RestResponseUtils {
             writer.flush();
             return writer.toString();
         } catch (Exception e) {
-            return ESTestCase.fail(e);
+            return fail(e);
         }
     }
+
+    public static <T extends ToXContent> T setUpXContentMock(T mock) {
+        try {
+            when(mock.toXContent(any(), any())).thenAnswer(
+                invocation -> asInstanceOf(XContentBuilder.class, invocation.getArgument(0)).startObject().endObject()
+            );
+        } catch (IOException e) {
+            fail(e);
+        }
+        return mock;
+    }
 }

x-pack/plugin/logstash/src/main/java/org/elasticsearch/xpack/logstash/rest/RestDeletePipelineAction.java

@@ -16,7 +16,6 @@ import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
 import org.elasticsearch.rest.action.RestActionListener;
-import org.elasticsearch.xcontent.XContentType;
 import org.elasticsearch.xpack.logstash.action.DeletePipelineAction;
 import org.elasticsearch.xpack.logstash.action.DeletePipelineRequest;
 import org.elasticsearch.xpack.logstash.action.DeletePipelineResponse;
@@ -49,7 +48,7 @@ public class RestDeletePipelineAction extends BaseRestHandler {
                 @Override
                 protected void processResponse(DeletePipelineResponse deletePipelineResponse) {
                     final RestStatus status = deletePipelineResponse.isDeleted() ? RestStatus.OK : RestStatus.NOT_FOUND;
-                    channel.sendResponse(new RestResponse(status, XContentType.JSON.mediaType(), BytesArray.EMPTY));
+                    channel.sendResponse(new RestResponse(status, RestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY));
                 }
             }
         );

x-pack/plugin/logstash/src/main/java/org/elasticsearch/xpack/logstash/rest/RestPutPipelineAction.java

@@ -16,7 +16,6 @@ import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
 import org.elasticsearch.rest.action.RestActionListener;
 import org.elasticsearch.xcontent.XContentParser;
-import org.elasticsearch.xcontent.XContentType;
 import org.elasticsearch.xpack.logstash.Pipeline;
 import org.elasticsearch.xpack.logstash.action.PutPipelineAction;
 import org.elasticsearch.xpack.logstash.action.PutPipelineRequest;
@@ -55,9 +54,9 @@ public class RestPutPipelineAction extends BaseRestHandler {
                 new PutPipelineRequest(id, content, request.getXContentType()),
                 new RestActionListener<>(restChannel) {
                     @Override
-                    protected void processResponse(PutPipelineResponse putPipelineResponse) throws Exception {
+                    protected void processResponse(PutPipelineResponse putPipelineResponse) {
                         channel.sendResponse(
-                            new RestResponse(putPipelineResponse.status(), XContentType.JSON.mediaType(), BytesArray.EMPTY)
+                            new RestResponse(putPipelineResponse.status(), RestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY)
                         );
                     }
                 }

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/rest/inference/RestUpdateTrainedModelDeploymentActionTests.java

@@ -12,19 +12,16 @@ import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.test.rest.FakeRestRequest;
 import org.elasticsearch.test.rest.RestActionTestCase;
-import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentType;
 import org.elasticsearch.xpack.core.ml.action.CreateTrainedModelAssignmentAction;
 import org.elasticsearch.xpack.core.ml.action.UpdateTrainedModelDeploymentAction;
 
-import java.io.IOException;
 import java.util.HashMap;
 
+import static org.elasticsearch.rest.RestResponseUtils.setUpXContentMock;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
-import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.when;
 
 public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestCase {
     public void testNumberOfAllocationInParam() {
@@ -37,7 +34,7 @@ public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestC
             assertEquals(request.getNumberOfAllocations().intValue(), 5);
 
             executeCalled.set(true);
-            return newMockResponse();
+            return setUpXContentMock(mock(CreateTrainedModelAssignmentAction.Response.class));
         }));
         var params = new HashMap<String, String>();
         params.put("number_of_allocations", "5");
@@ -60,7 +57,7 @@ public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestC
             assertEquals(request.getNumberOfAllocations().intValue(), 6);
 
             executeCalled.set(true);
-            return newMockResponse();
+            return setUpXContentMock(mock(CreateTrainedModelAssignmentAction.Response.class));
         }));
 
         final String content = """
@@ -73,16 +70,4 @@ public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestC
         dispatchRequest(inferenceRequest);
         assertThat(executeCalled.get(), equalTo(true));
     }
-
-    private static CreateTrainedModelAssignmentAction.Response newMockResponse() {
-        final var response = mock(CreateTrainedModelAssignmentAction.Response.class);
-        try {
-            when(response.toXContent(any(), any())).thenAnswer(
-                invocation -> asInstanceOf(XContentBuilder.class, invocation.getArgument(0)).startObject().endObject()
-            );
-        } catch (IOException e) {
-            fail(e);
-        }
-        return response;
-    }
 }