[rust] Umbrella: fluss-rust client support required by the Fluss REST Gateway

### Search before asking

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

### Motivation

The Fluss REST Gateway (FIP-49, link https://cwiki.apache.org/confluence/spaces/FLUSS/pages/444334561/FIP-49+Fluss+Gateway+REST+API) is a standalone stateless service that exposes REST write / lookup / metadata APIs on top of Fluss. It uses **fluss-rust as a standard Fluss client** — no server-side changes are required — so the gateway's production readiness is directly bounded by the fluss-rust client.

This is the umbrella issue tracking the fluss-rust work that the gateway depends on. Concrete gaps are filed as sub-issues and linked below as they are identified; the gateway FIP references this issue as the single entry point.

### Sub-issues

- [x] #3855 Support cooperative KV backpressure (client side of #3463)
- [ ] #3861
- [ ] #3959
- [ ] #4064

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start with FIP-49 and the linked sub-issues #3855, #3861, #3959, and #4064 to understand the concrete fluss-rust gaps required by the Fluss REST Gateway. This umbrella issue is done when the required client-support sub-issues are completed and the gateway's stated dependencies are met.

Written by the indexing model from the issue text.