Fix UT

Signed-off-by: Peng Huo <penghuo@gmail.com>

Author: penghuo

protocol/src/test/java/org/opensearch/sql/protocol/response/format/YamlResponseFormatterTest.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.protocol.response.format;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+import org.junit.jupiter.api.DisplayNameGeneration;
+import org.junit.jupiter.api.DisplayNameGenerator;
+import org.junit.jupiter.api.Test;
+import org.opensearch.sql.utils.YamlFormatter;
+
+@DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
+class YamlResponseFormatterTest {
+
+  private final YamlResponseFormatter<Object> formatter =
+      new YamlResponseFormatter<>() {
+        @Override
+        protected Object buildYamlObject(Object response) {
+          // Pass-through for testing: return the response directly
+          return response;
+        }
+      };
+
+  @Test
+  void content_type_matches_yaml() {
+    assertEquals(YamlResponseFormatter.CONTENT_TYPE, formatter.contentType());
+  }
+
+  @Test
+  void formats_response_via_yaml_formatter() {
+    Map<String, Object> payload = new LinkedHashMap<>();
+    payload.put("b", 2);
+    payload.put("a", "1");
+
+    String expected = YamlFormatter.formatToYaml(payload);
+    String actual = formatter.format(payload);
+
+    assertEquals(expected, actual);
+  }
+
+  @Test
+  void formats_throwable_via_yaml_formatter() {
+    Exception e = new Exception("boom", new RuntimeException("root-cause"));
+
+    String expected = YamlFormatter.formatToYaml(e);
+    String actual = formatter.format(e);
+
+    assertEquals(expected, actual);
+  }
+}