Fix race in agentless config source listener test

scheduledPollContinuesAfterListenerRuntimeException waited on
FakeClient.calls, which is incremented when a request starts rather than
when it completes. The barrier therefore released as soon as the second
poll began, letting the assertion race the poll thread that applies the
configuration and notifies the listener.

Wait on a CountDownLatch counted down by the listener itself, so the
second notification is guaranteed to have happened before the
assertions run.

Environment: Datadog workspace

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Co-authored-by: vickie.fridge <vickie.fridge@datadoghq.com>

Author: vjfridge

products/feature-flagging/feature-flagging-lib/src/test/java/com/datadog/featureflag/AgentlessConfigurationSourceTest.java

@@ -920,17 +920,22 @@ void scheduledPollContinuesAfterListenerRuntimeException() throws Exception {
             client,
             Executors.newSingleThreadScheduledExecutor());
     final AtomicInteger listenerCalls = new AtomicInteger();
+    // Wait on the listener rather than on FakeClient.calls: the call counter is incremented when
+    // a request starts, so it reaches 2 before the second configuration has been applied.
+    final CountDownLatch listenerNotified = new CountDownLatch(2);
     final FeatureFlaggingGateway.ConfigListener flakyListener =
         configuration -> {
-          if (listenerCalls.incrementAndGet() == 1) {
+          final int call = listenerCalls.incrementAndGet();
+          listenerNotified.countDown();
+          if (call == 1) {
             throw new IllegalStateException("listener rejected first configuration");
           }
         };
     FeatureFlaggingGateway.addConfigListener(flakyListener);
 
     try {
       service.init();
-      awaitCalls(client, 2);
+      assertTrue(listenerNotified.await(5, TimeUnit.SECONDS));
 
       assertEquals(2, listenerCalls.get());
       assertNull(client.requests.get(1).etag);