Skip refreshIndex in serverless mode to fix 404 on _refresh endpoint (opensearch-project#690)

Signed-off-by: Sotaro Hikita <bering1814@gmail.com>

Author: lawofcycles

mr/src/main/java/org/opensearch/hadoop/rest/RestClient.java

@@ -309,6 +309,12 @@ public void refresh(Resource resource) {
     }
 
     public void refreshIndex(String index) {
+        if (settings.getServerlessMode()) {
+            if (LOG.isDebugEnabled()) {
+                LOG.debug("Serverless mode - skipping refreshIndex (not supported in serverless)");
+            }
+            return;
+        }
         execute(POST, index + "/_refresh");
     }