Unified Testing for Lake Storage Union Read to Reduce Code Duplication

### Search before asking

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

### Description

Both Iceberg and Paimon have their own integration tests for lake storage union reads, leading to significant code duplication. We can create a new module to unified integration tests for this functionality.

### Willingness to contribute

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

Locate the existing Iceberg and Paimon integration tests for lake storage union reads and compare their shared setup and coverage. Create a unified testing approach that covers both implementations without duplicating the same cases; done means the union-read behavior remains covered for both lake storage paths.

Written by the indexing model from the issue text.