Add End-To-End Tests

### Search before asking

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

### Description

We need to add more tests, especially end-to-end tests that set up a distributed Fluss cluster for verification. This is crucial because nearly all of the issues we encounter are related to class loading.

### Willingness to contribute

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

The issue names no files, tests, or entry points; start by surveying Fluss's existing test layout and distributed-cluster setup. Define reproducible end-to-end scenarios that exercise class loading, and consider the work done when those scenarios run against a distributed cluster and verify the intended behavior.

Written by the indexing model from the issue text.