Fix RestCancellableNodeClientTests.testConcurrentExecuteAndClose()

Fixes a race condition in testConcurrentExecuteAndClose() where
awaitClose() is called before addCloseListener() has been called,
resulting in a situation where closeListener is never completed and
the closeLatch is never pulled.

Closes #129121

Author: JeremyDahlgren

server/src/test/java/org/elasticsearch/rest/action/RestCancellableNodeClientTests.java

@@ -303,6 +303,10 @@ public void addCloseListener(ActionListener<Void> listener) {
             // if the channel is already closed, the listener gets notified immediately, from the same thread.
             if (open.get() == false) {
                 listener.onResponse(null);
+                // Handle scenario where awaitClose() was called before any calls to addCloseListener(), this ensures closeLatch is pulled.
+                if (closeListener.isDone() == false) {
+                    closeListener.onResponse(ActionListener.noop());
+                }
             } else {
                 assertFalse("close listener already set, only one is allowed!", closeListener.isDone());
                 closeListener.onResponse(ActionListener.assertOnce(listener));