Flaky-test: ExclusiveProducerTest.exclusiveWithConsumers

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Example failures

- [2024-11-28T22:40:38.6738558Z](https://github.com/apache/pulsar/actions/runs/12076063420/job/33677065208#step:10:1198)
- [2024-11-26T21:16:26.3249055Z](https://github.com/apache/pulsar/actions/runs/12023175603/job/33566078982#step:11:1240)

### Exception stacktrace

```
java.util.concurrent.ExecutionException: io.netty.channel.StacklessClosedChannelException
at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:396)
at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2073)
at org.apache.pulsar.broker.service.ExclusiveProducerTest.simpleTest(ExclusiveProducerTest.java:215)
at org.apache.pulsar.broker.service.ExclusiveProducerTest.exclusiveWithConsumers(ExclusiveProducerTest.java:423)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at org.testng.internal.invokers.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:139)
at org.testng.internal.invokers.InvokeMethodRunnable.runOne(InvokeMethodRunnable.java:47)
at org.testng.internal.invokers.InvokeMethodRunnable.call(InvokeMethodRunnable.java:76)
at org.testng.internal.invokers.InvokeMethodRunnable.call(InvokeMethodRunnable.java:11)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
at java.base/java.lang.Thread.run(Thread.java:1583)
Caused by: io.netty.channel.StacklessClosedChannelException
at io.netty.channel.AbstractChannel$AbstractUnsafe.write(Object, ChannelPromise)(Unknown Source)
```

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with ExclusiveProducerTest.exclusiveWithConsumers and its simpleTest helper, especially the reported lines 423 and 215. Reproduce the test failure using the linked GitHub Actions runs or the project's test command, then trace the StacklessClosedChannelException. Done means the test no longer flakes and still passes its exclusive-producer and consumer checks.

Written by the indexing model from the issue text.