[Umbrella] Improve Test Code Coverage for Ignored Classes

### Search before asking

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

### Description

Our current test code coverage is below the desired threshold, leaving parts of the codebase untested and potentially prone to bugs or regressions. Increasing test coverage is critical to ensuring the reliability, maintainability, and long-term stability of our application.

We have temporarily ignored a lot of classes in the `test-coverage`: https://github.com/alibaba/fluss/blob/main/fluss-test-coverage/pom.xml#L247

We need to improve the code coverage by adding unit tests and integration tests around the classes, and remove them from the list.

The purpose of increasing test coverage is to improve code stability. Therefore, when adding tests, it is important to understand the logic of the classes and code, and to add effective tests (UT or ITCase) that align with the code logic, rather than simply adding mock-based tests solely aimed at increasing coverage.

If you're interested in this issue, feel free to create a sub-task and take it on.

The following is a reference PR on how to improve test coverage：
https://github.com/alibaba/fluss/pull/856

If you hope to focus on this job and communicate more efficiently, you're welcome to join the Fluss Slack channel: https://alibaba.github.io/fluss-docs/community/welcome/#slack

### Willingness to contribute

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