Fix stack trace in ActionListener#assertOnce (elastic#124672)

In elastic#112380 we changed this `assert` to yield a `String` on failure
rather than the original `ElasticsearchException`, which means we don't
see the original completion's stack trace any more. This commit
reinstates the lost stack trace.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/action/ActionListener.java

@@ -401,7 +401,9 @@ static <Response> ActionListener<Response> assertOnce(ActionListener<Response> d
 
                 private void assertFirstRun() {
                     var previousRun = firstCompletion.compareAndExchange(null, new ElasticsearchException("executed already"));
-                    assert previousRun == null : "[" + delegate + "] " + previousRun; // reports the stack traces of both completions
+                    assert previousRun == null
+                        // reports the stack traces of both completions
+                        : new AssertionError("[" + delegate + "]", previousRun);
                 }
 
                 @Override