[Bug] Acknowledge might fail with ResultAlreadyClosed during unload

### Search before asking

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

### Version

main

### Minimal reproduce step

Enable `ackReceipt` for https://github.com/apache/pulsar-client-cpp/blob/main/tests/extensibleLM/ExtensibleLoadManagerTest.cc

```c++
ConsumerConfiguration consumerConf;
consumerConf.setAckReceiptEnabled(true);
Consumer consumer;
Result consumerResult = client.subscribe(topicName, "sub", consumerConf, consumer);
```

### What did you expect to see?

The test passed

### What did you see instead?

```
2024-02-27 21:51:30.938 WARN [0x16b36b000] AckGroupingTracker:93 | Connection is not ready, ACK failed for [(14,0,-1,-1)]
2024-02-27 21:51:30.938 INFO [0x16b96f000] ExtensibleLoadManagerTest:145 | acked i: 0 81 ms
/Users/xuyunze/github.com/bewaremypower/pulsar-client-cpp/tests/extensibleLM/ExtensibleLoadManagerTest.cc:147: Failure
Expected equality of these values:
ackResult
Which is: AlreadyClosed
ResultOk
Which is: Ok
```

Note: I changed the log level of `AckGroupingTracker:93` to warn locally.

### Anything else?

When `HandlerBase::getCnx()` returns a null `weak_ptr`, it's retryable if the state is not `Closing` or `Closed`. We should schedule the flush again.

### Are you willing to submit a PR?

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

Bắt đầu với tests/extensibleLM/ExtensibleLoadManagerTest.cc, đặc biệt là phần thiết lập ackReceipt và assertion bị lỗi quanh dòng 147. Theo dõi AckGroupingTracker và HandlerBase::getCnx() để hiểu đường dẫn unload và điều kiện retry. Hoàn tất khi test chạy thành công với ackResult bằng ResultOk thay vì AlreadyClosed khi client unload.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.