[server] Follower may stop syncing from the leader due to unknown network issues

### Search before asking

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

### Fluss version

main (development)

### Please describe the bug 🐞

Follower may stop syncing from the leader due to unknown network issues. The network IP and port are both correct, but synchronization fails. After restarting the pod, it recovers—suspected to be a network-layer issue, root cause under investigation:

### Solution

_No response_

### Are you willing to submit a PR?

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

No files, tests, or entry points are identified. Start by examining the server follower-to-leader synchronization path and the attached failure evidence, then reproduce the stop and compare behavior before and after a pod restart. Done means identifying and fixing the network-related cause and verifying synchronization recovers without restarting.

Written by the indexing model from the issue text.