[client] Is there any reason why primitives are not supported in converters?

### Search before asking

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

### Motivation

see header, otherwise would make sense to support it

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

No file, test, or specific converter is named. Start by locating the client converter implementations and their existing tests, then determine which primitive types are currently rejected and what behavior existing converters establish. Done means the intended primitive types are supported with matching test coverage.

Written by the indexing model from the issue text.