Add sleep to stress test acker spin loop

The acker thread in testHighConcurrencyStress() spun in a tight
loop with no sleep when there was nothing new to acknowledge,
wasting CPU. Add Thread.sleep(1) in the else branch, matching
the pattern already used in testConcurrentAddAndCumulativeAck().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mtopolnik

core/src/test/java/io/questdb/client/test/cutlass/qwp/client/InFlightWindowTest.java

@@ -612,8 +612,9 @@ public void testHighConcurrencyStress() throws Exception {
                     if (highest > lastAcked) {
                         window.acknowledgeUpTo(highest);
                         lastAcked = highest;
+                    } else {
+                        Thread.sleep(1);
                     }
-                    // No sleep - maximum contention
                 }
             } catch (Throwable t) {
                 error.set(t);