Support FixedSizeBinary in schema conversion in python

### Search before asking

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


### Description

The Python binding currently rejects PyArrow fixed-size binary fields (`pa.binary(n)`), even though fixed-size binary is supported in Fluss Java and Rust core (`BINARY(n)`).


### Willingness to contribute

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