[Bug] unacked message count is zero when using exclusive subscription.

### Search before asking

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

### Read release policy

- [x] I understand that unsupported versions don't get bug fixes. I will attempt to reproduce the issue on a supported version of Pulsar client and Pulsar broker.

### Version

Pulsar: master

### Minimal reproduce step

```java
@Test
public void testUnackedMessages() throws Exception {
String topicName = "testUnackedMessages";
String subscriptionName = "my-sub";
String serviceUrl = "pulsar://localhost:6650";

@Cleanup PulsarClient pulsarClient = PulsarClient.builder().serviceUrl(serviceUrl).build();

@Cleanup Consumer consumer =
pulsarClient.newConsumer().topic(topicName).subscriptionName(subscriptionName)
.subscriptionType(SubscriptionType.Exclusive).messageListener((c, message) -> {
// noop
}).subscribe();

@Cleanup Producer producer = pulsarClient.newProducer().topic(topicName).create();
producer.send("1".getBytes());

TopicStats stats = admin.topics().getStats(topicName);
assertThat(stats.getSubscriptions().get(subscriptionName).getUnackedMessages()).isNotEqualTo(0);
}
```

### What did you expect to see?

unackedMessages is not equal to 0.

### What did you see instead?

unackedMessages is equal to 0.

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by running the Java reproduction in the issue and inspect the TopicStats subscription entry for an exclusive subscription. Trace how unackedMessages is calculated and reported for this case; the work is done when the provided assertion observes a nonzero count without breaking other subscription types.

Written by the indexing model from the issue text.