[Bug] Proxy remoting heartbeat should reject empty request bodies

### What happened?

`ClientManagerActivity.heartBeat` decodes the remoting request body with `HeartbeatData.decode(request.getBody(), HeartbeatData.class)` and immediately dereferences the result. `RemotingSerializable.decode` returns `null` for a null body, so a malformed `HEART_BEAT` request without a body can hit a `NullPointerException` and be reported as an internal proxy error.

### Why this matters

The Proxy client-management path is part of the Studio/Proxy Admin foundation. Malformed client heartbeat frames should be rejected with a clear response code and remark instead of surfacing as an internal exception.

### Expected behavior

Proxy should return a deterministic `INVALID_PARAMETER` response for empty or undecodable heartbeat bodies and must not register producer or consumer channels for that request.

### Scope

RocketMQ Studio track 2 / Proxy client management hardening.

Start at ClientManagerActivity.heartBeat and trace how HeartbeatData.decode and RemotingSerializable.decode handle empty or undecodable request bodies. Add or update coverage for malformed HEART_BEAT requests, then verify the response is INVALID_PARAMETER and that no producer or consumer channel is registered.

Written by the indexing model from the issue text.