Fix SemanticHighlighterExtBuilderTests.toXContent

SearchExtBuilder expects toXContent to produce their encloding field
name.
Add test to ensure we can do a round trip with XContent.

Fixes #1906.

Signed-off-by: David Causse <dcausse@wikimedia.org>

Author: nomoa

src/main/java/org/opensearch/neuralsearch/query/ext/SemanticHighlighterExtBuilder.java

@@ -4,17 +4,17 @@
  */
 package org.opensearch.neuralsearch.query.ext;
 
-import java.io.IOException;
-import java.util.Locale;
-import java.util.Objects;
-
 import org.opensearch.core.common.io.stream.StreamInput;
 import org.opensearch.core.common.io.stream.StreamOutput;
 import org.opensearch.core.xcontent.XContentBuilder;
 import org.opensearch.core.xcontent.XContentParser;
 import org.opensearch.neuralsearch.highlight.SemanticHighlightingConstants;
 import org.opensearch.search.SearchExtBuilder;
 
+import java.io.IOException;
+import java.util.Locale;
+import java.util.Objects;
+
 /**
  * Search request ext builder that opts a request into batch semantic highlighting.
  *
@@ -61,7 +61,7 @@ public void writeTo(StreamOutput out) throws IOException {
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
-        return builder.value(enabled);
+        return builder.field(NAME, enabled);
     }
 
     @Override

src/test/java/org/opensearch/neuralsearch/query/ext/SemanticHighlighterExtBuilderTests.java

@@ -4,18 +4,24 @@
  */
 package org.opensearch.neuralsearch.query.ext;
 
-import java.io.IOException;
-import java.util.List;
-
 import org.opensearch.common.io.stream.BytesStreamOutput;
+import org.opensearch.common.xcontent.XContentType;
 import org.opensearch.core.ParseField;
+import org.opensearch.core.common.bytes.BytesReference;
 import org.opensearch.core.common.io.stream.NamedWriteableRegistry;
 import org.opensearch.core.common.io.stream.StreamInput;
+import org.opensearch.core.xcontent.MediaType;
 import org.opensearch.core.xcontent.NamedXContentRegistry;
+import org.opensearch.core.xcontent.ToXContentObject;
+import org.opensearch.core.xcontent.XContentBuilder;
 import org.opensearch.core.xcontent.XContentParser;
 import org.opensearch.search.SearchExtBuilder;
 import org.opensearch.test.OpenSearchTestCase;
 
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.List;
+
 public class SemanticHighlighterExtBuilderTests extends OpenSearchTestCase {
 
     @Override
@@ -57,6 +63,14 @@ public void testRoundTripStreamingFalse() throws IOException {
         roundTrip(false);
     }
 
+    public void testRoundTripXContentTrue() throws IOException {
+        roundTripXContent(true);
+    }
+
+    public void testRoundTripXContentFalse() throws IOException {
+        roundTripXContent(false);
+    }
+
     public void testParseBooleanTrue() throws IOException {
         SemanticHighlighterExtBuilder result = parseValue("true");
         assertTrue(result.isEnabled());
@@ -97,6 +111,28 @@ private void roundTrip(boolean value) throws IOException {
         }
     }
 
+    private void roundTripXContent(boolean value) throws IOException {
+        SemanticHighlighterExtBuilder original = new SemanticHighlighterExtBuilder(value);
+        MediaType xContentType = randomFrom(XContentType.values());
+
+        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+        XContentBuilder builder = xContentType.contentBuilder(outputStream);
+        builder.startObject();
+        original.toXContent(builder, ToXContentObject.EMPTY_PARAMS);
+        builder.endObject();
+        BytesReference originalBytes = BytesReference.bytes(builder);
+
+        try (XContentParser parser = this.createParser(xContentType.xContent(), originalBytes)) {
+            assertEquals(XContentParser.Token.START_OBJECT, parser.nextToken());
+            assertEquals(XContentParser.Token.FIELD_NAME, parser.nextToken());
+            assertEquals(SemanticHighlighterExtBuilder.NAME, parser.currentName());
+            parser.nextToken(); // advance to the value
+            SearchExtBuilder deserialized = parser.namedObject(SearchExtBuilder.class, SemanticHighlighterExtBuilder.NAME, null);
+            assertEquals(XContentParser.Token.END_OBJECT, parser.nextToken());
+            assertEquals(original, deserialized);
+        }
+    }
+
     private SemanticHighlighterExtBuilder parseValue(String json) throws IOException {
         XContentParser parser = createParser(org.opensearch.common.xcontent.XContentType.JSON.xContent(), json);
         // advance past START_TOKEN to the value