[server] Push partition filtering to the server-side to reduce network bandwidth usage

### Search before asking

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

### Motivation

Push partition filtering to the server-side to reduce network bandwidth usage

we can optimize on the ZK side: when the ResolvedPartitionSpec is a complete partition spec, we can directly retrieve the partition instead of scanning all partitions. This can satisfy some scenarios.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by tracing ResolvedPartitionSpec through the server-side partition lookup and ZK-related code paths. Confirm how complete partition specifications are currently handled, then determine the tests or measurements needed to show that direct retrieval reduces scanning and network usage.

Written by the indexing model from the issue text.