Mark grammar API as experimental

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

plugin/src/main/java/org/opensearch/sql/plugin/rest/RestPPLGrammarAction.java

@@ -12,6 +12,7 @@
 import java.io.IOException;
 import java.util.List;
 import lombok.extern.log4j.Log4j2;
+import org.opensearch.common.annotation.ExperimentalApi;
 import org.opensearch.core.rest.RestStatus;
 import org.opensearch.core.xcontent.XContentBuilder;
 import org.opensearch.rest.BaseRestHandler;
@@ -21,7 +22,12 @@
 import org.opensearch.sql.ppl.autocomplete.PPLGrammarBundleBuilder;
 import org.opensearch.transport.client.node.NodeClient;
 
-/** REST handler for {@code GET /_plugins/_ppl/_grammar}. */
+/*
+ * REST handler for {@code GET /_plugins/_ppl/_grammar}.
+ *
+ * @opensearch.experimental
+ */
+@ExperimentalApi
 @Log4j2
 public class RestPPLGrammarAction extends BaseRestHandler {