FlussConnection cann't get table while the request server not ready

### Search before asking

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

### Fluss version

0.8.0 (latest release)

### Please describe the bug 🐞

```
2025-12-03 18:11:40:
java.util.concurrent.CompletionException: org.apache.fluss.exception.NetworkException: org.apache.fluss.exception.DisconnectException: Cannot send request to server xxxx:9123 (id: ts-2, rack: null) because it is disconnected.
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:331)
at java.base/java.util.concurrent.CompletableFuture.uniApplyNow(CompletableFuture.java:670)
at java.base/java.util.concurrent.CompletableFuture.uniApplyStage(CompletableFuture.java:658)
at java.base/java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:2100)
at org.apache.fluss.client.admin.FlussAdmin.getTableInfo(FlussAdmin.java:264)
at org.apache.fluss.client.FlussConnection.getTable(FlussConnection.java:106)
at org.apache.fluss.flink.source.reader.FlinkSourceSplitReader.(FlinkSourceSplitReader.java:129)
at org.apache.fluss.flink.source.reader.FlinkSourceReader.lambda$new$0(FlinkSourceReader.java:69)
at org.apache.flink.connector.base.source.reader.fetcher.SplitFetcherManager.createSplitFetcher(SplitFetcherManager.java:259)
at org.apache.flink.connector.base.source.reader.fetcher.SingleThreadFetcherManager.addSplits(SingleThreadFetcherManager.java:148)
at org.apache.flink.connector.base.source.reader.SourceReaderBase.addSplits(SourceReaderBase.java:341)
at org.apache.flink.streaming.api.operators.SourceOperator.handleAddSplitsEvent(SourceOperator.java:694)
at org.apache.flink.streaming.api.operators.SourceOperator.handleOperatorEvent(SourceOperator.java:638)
at org.apache.flink.streaming.runtime.tasks.OperatorEventDispatcherImpl.dispatchEventToHandlers(OperatorEventDispatcherImpl.java:72)
at org.apache.flink.streaming.runtime.tasks.RegularOperatorChain.dispatchOperatorEvent(RegularOperatorChain.java:80)
at org.apache.flink.streaming.runtime.tasks.StreamTask.lambda$dispatchOperatorEvent$24(StreamTask.java:1891)
at org.apache.flink.streaming.runtime.tasks.StreamTaskActionExecutor$1.runThrowing(StreamTaskActionExecutor.java:50)
at org.apache.flink.streaming.runtime.tasks.mailbox.Mail.run(Mail.java:101)
at org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor.runMail(MailboxProcessor.java:414)
at org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor.processMailsWhenDefaultActionUnavailable(MailboxProcessor.java:383)
at org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor.processMail(MailboxProcessor.java:368)
at org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor.runMailboxLoop(MailboxProcessor.java:229)
at org.apache.flink.streaming.runtime.tasks.StreamTask.runMailboxLoop(StreamTask.java:1202)
at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:1146)
at org.apache.flink.runtime.taskmanager.Task.runWithSystemExitMonitoring(Task.java:976)
at org.apache.flink.runtime.taskmanager.Task.restoreAndInvoke(Task.java:955)
at org.apache.flink.runtime.taskmanager.Task.doRun(Task.java:768)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:580)
at java.base/java.lang.Thread.run(Thread.java:991)
Caused by: org.apache.fluss.exception.NetworkException: org.apache.fluss.exception.DisconnectException: Cannot send request to server xxx:9123 (id: ts-2, rack: null) because it is disconnected.
Caused by: org.apache.fluss.exception.DisconnectException: Cannot send request to server xxx:9123 (id: ts-2, rack: null) because it is disconnected.
```

### Solution

_No response_

### Are you willing to submit a PR?

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

Start with FlussConnection.getTable and FlussAdmin.getTableInfo, the entry points shown in the stack trace, and reproduce the request while the server is not ready. Trace how the disconnect is propagated and clarify the expected behavior for this startup race; done should be demonstrated by a regression test covering table access before the request server is ready.

Written by the indexing model from the issue text.