ESQL: Stabilize heap attack test (elastic#132045)

In some contexts ESQL's HeapAttack tests were failing because the
TranslogWriter keeps a map with the contents of the documents in it
around. But only when assertions are enabled. Fun. This flushes the map
away, preventing the odd OOM we were seeing in the HeapAttack tests.

Author: nik9000

test/external-modules/esql-heap-attack/src/javaRestTest/java/org/elasticsearch/xpack/esql/heap_attack/HeapAttackIT.java

@@ -894,7 +894,6 @@ private void initGiantTextField(int docs) throws IOException {
                 }
             }
         }
-
         logger.info("loading many documents with one big text field - docs per bulk {}", docsPerBulk);
 
         int fieldSize = Math.toIntExact(ByteSizeValue.ofMb(5).getBytes());
@@ -1064,6 +1063,15 @@ private void bulk(String name, String bulk) throws IOException {
         );
         Response response = client().performRequest(request);
         assertThat(entityAsMap(response), matchesMap().entry("errors", false).extraOk());
+
+        /*
+         * Flush after each bulk to clear the test-time seenSequenceNumbers Map in
+         * TranslogWriter. Without this the server will OOM from time to time keeping
+         * stuff around to run assertions on.
+         */
+        request = new Request("POST", "/" + name + "/_flush");
+        response = client().performRequest(request);
+        assertThat(entityAsMap(response), matchesMap().entry("_shards", matchesMap().extraOk().entry("failed", 0)).extraOk());
     }
 
     private void initIndex(String name, String bulk) throws IOException {