[Umbrella] Subissue Improve io.* 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.io.* to increase the coverage.

### Willingness to contribute

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

Start by inspecting the com.alibaba.fluss.io.* package and the coverage configuration to identify the ignored classes. Add focused unit or integration tests for those classes, then verify that coverage improves and the tests pass.

Written by the indexing model from the issue text.