Fix SimpleNestedIT.testExplain flaky test

Signed-off-by: Neetika Singhal <[email protected]>

server/src/internalClusterTest/java/org/opensearch/search/nested/SimpleNestedIT.java

@@ -487,6 +487,13 @@ public void testExplain() throws Exception {
             )
             .setRefreshPolicy(IMMEDIATE)
             .get();
+
+        // refresh is important to enforce creation of a segment for document ingest above
+        // and prevent merging with deleted segemnts created by the index random function
+        // this ensures that the range doesnt change for the child docs asserted below
+        // ref: https://github.com/opensearch-project/OpenSearch/issues/11413
+        refresh("test");
+
         indexRandomForConcurrentSearch("test");
 
         SearchResponse searchResponse = client().prepareSearch("test")