Add artificial delay for deterministic race condition reproduction

Add Thread.sleep(50) inside the async handler's Future to widen the race
window between Future continuation cleanup and span completion. Without
the .recover fix, this delay causes the failed-Future trace-drop to
trigger reliably. With .recover, the delay is harmless because the
Future always succeeds.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bm1549

dd-java-agent/instrumentation/pekko/pekko-http-1.0/src/baseTest/scala/PekkoHttpTestWebServer.scala

@@ -274,6 +274,10 @@ object PekkoHttpTestWebServer {
       ec: ExecutionContext
   ): HttpRequest => Future[HttpResponse] = { request =>
     Future {
+      // Deterministic reproduction: delay async execution to widen the race window
+      // between Future continuation cleanup and span completion. Without .recover,
+      // this delay makes the failed-Future trace-drop race condition trigger reliably.
+      Thread.sleep(50)
       syncHandler(request)
     }.recover { case e: Exception =>
       // Recover from exceptions to return a proper HTTP response instead of a