Stabilize CancellableTasksIT#testRemoveBanParentsOnDisconnect (#131858)

Ensure that the cluster has a stably elected master node
when the test completes so that the post-test tear-down
check for cluster state consistency can succeed.

Closes #131562

Author: DiannaHohensee

muted-tests.yml

@@ -449,9 +449,6 @@ tests:
 - class: org.elasticsearch.xpack.search.CrossClusterAsyncSearchIT
   method: testCancellationViaTimeoutWithAllowPartialResultsSetToFalse
   issue: https://github.com/elastic/elasticsearch/issues/131248
-- class: org.elasticsearch.action.admin.cluster.node.tasks.CancellableTasksIT
-  method: testRemoveBanParentsOnDisconnect
-  issue: https://github.com/elastic/elasticsearch/issues/131562
 - class: org.elasticsearch.xpack.esql.action.CrossClusterQueryWithPartialResultsIT
   method: testPartialResults
   issue: https://github.com/elastic/elasticsearch/issues/131481

server/src/internalClusterTest/java/org/elasticsearch/action/admin/cluster/node/tasks/CancellableTasksIT.java

@@ -363,6 +363,10 @@ public void testRemoveBanParentsOnDisconnect() throws Exception {
             allowEntireRequest(rootRequest);
             cancelFuture.actionGet();
             ensureBansAndCancellationsConsistency();
+            // This method will call the cluster health API with wait_for_tasks set to Priority#LANGUID. This will ensure that all tasks
+            // currently submitted to the master service (included re-election after a disconnect) are processed before returning. This
+            // ensures cluster stability for the post-test consistency checks.
+            ensureStableCluster(nodes.size());
         }
     }