Fix 'operation buffered during sync' test to actually test buffering

The test introduced in 6b1c2de was not actually exercising the
buffered-during-sync code path. It called counter.increment(5) and
immediately completed the sync, but the ACK consistently arrived
after sync completion, so the operation was applied directly rather
than being buffered. The test passed even with the buffering
implementation disabled.

Fix by using an ACK interceptor to deterministically control when
the ACK is delivered: intercept it, release it, then yield to the
event loop so publishAndApply enters the sync-wait before we
complete the sync.

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

Author: lawrence-forooghian

test/realtime/liveobjects.test.js

@@ -8815,10 +8815,24 @@ define(['ably', 'shared_helper', 'chai', 'liveobjects', 'liveobjects_helper'], f
             // inject an ATTACHED with HAS_OBJECTS to trigger SYNCING state
             await injectAttachedMessage(helper, channel, 1 << 7); // HAS_OBJECTS flag is bit 7
 
+            // set up ACK interceptor so we can control when the ACK is delivered
+            const interceptor = createAckInterceptor(helper, client);
+
             // perform an increment while in SYNCING state
-            // the ACK will be buffered until sync completes
             const incrementPromise = counter.increment(5);
 
+            // wait for the ACK to be intercepted, then release it.
+            // this lets publishAndApply proceed past publish into the sync-wait.
+            await interceptor.waitForAck();
+            interceptor.releaseAll();
+
+            // yield to the event loop so publishAndApply reaches the sync wait.
+            // (we use the same mechanism as the 'publishAndApply rejects when channel
+            // state changes during sync wait' test, which validates this yield is
+            // sufficient — if publishAndApply had not yet entered the sync-wait, the
+            // channel state change would not cause it to reject.)
+            await new Promise((resolve) => setTimeout(resolve, 0));
+
             // complete the sync sequence
             await objectsHelper.processObjectStateMessageOnChannel({
               channel,