[Umbrella] Subissue Improve protogen.* test coverage

### Search before asking

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

### Description

Currently, we ignore a lot of classes to make the code coverage pass. We need to add unit tests or integration tests around those classes to improve the coverage.
In this issue, it aims to cover `com.alibaba.fluss.protogen.*` to increase the coverage.

### Willingness to contribute

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

Start by locating the com.alibaba.fluss.protogen.* classes currently ignored by coverage and review the existing unit and integration test structure. Add tests around the uncovered classes, then run the relevant test and coverage checks; done means the protogen.* coverage is improved without relying on those classes being ignored.

Written by the indexing model from the issue text.