[rust] Dedup KV value decode between lookup and batch scanner

### Search before asking

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

### Description

LookupResult::to_record_batch and the limit scanner's KV decode do the same thing — [schema_id|value] -> FixedSchemaDecoder -> RecordBatch - with separate decoder caches and a duplicated read_schema_id. Extract one shared helper, the future snapshot scanner will want it too.

Touches the lookup hot path, so do it carefully on its own.

### Willingness to contribute

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