Support Array/NestedRow/Map types in union read

### Search before asking

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

### Motivation

Enable union read to correctly handle complex types when reading from both local and remote storage.

### Solution

- Ensure union read can merge data with complex types from different sources
- Implement correct schema resolution for complex types in union read
- Add tests for union read with complex types

### Anything else?

_No response_

### Willingness to contribute

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

Start by locating the union read implementation and its schema-resolution path for local and remote storage. Review how Array, NestedRow, and Map types are represented and merged across sources. Add focused tests for union reads involving these complex types; done means the tests pass with correctly resolved schemas and merged data.

Written by the indexing model from the issue text.