feat: feat(cluster-protocol): add the Rust WebSocket binding and cloud data-plane contract

[tomdps]

Closes #651

[greptile-apps]

Greptile Summary

Adds cooperative WebSocket request cancellation with ownership-safe request-ID reuse.

• Registers per-request cancellation signals before spawning unary request tasks.
• Races unary dispatch against $/cancelRequest and suppresses cancelled responses.
• Adds unit and integration coverage for registration, cleanup, cancellation, and same-ID reuse.

Confidence Score: 5/5

The PR appears safe to merge, with cancellation registration, cleanup, and request-ID reuse handled consistently.

Cooperative cancellation always converges on shared cleanup, registry removal is ownership-checked against same-ID reuse, and current production backend operations expose no cancellation-sensitive partial mutation.

Important Files Changed

Filename	Overview
crates/openengine-cluster-server/src/websocket.rs	Replaces abort-handle cancellation with cooperative notification, synchronous registration, and ownership-checked cleanup without an identified correctness defect.
crates/openengine-cluster-server/src/websocket/tests.rs	Adds focused white-box tests for registration ordering, completion cleanup, and protection of newer same-ID registrations.
crates/openengine-cluster-server/tests/websocket.rs	Adds wire-level tests proving cancelled requests emit no response and request IDs remain reusable and independently cancellable.

Sequence Diagram

sequenceDiagram
    participant Client
    participant WebSocket
    participant Registry
    participant Task
    participant Backend
    Client->>WebSocket: Unary request (id)
    WebSocket->>Registry: Register Notify under id
    WebSocket->>Task: Spawn request task
    Task->>Backend: dispatch(request)
    Client->>WebSocket: $/cancelRequest(id)
    WebSocket->>Registry: Lookup Notify
    Registry-->>Task: Notify cancellation
    Task--xBackend: Drop dispatch future
    Task->>Registry: Remove owned entry
    Note over Task,Client: Cancelled request emits no response
    Client->>WebSocket: Reuse id
    WebSocket->>Registry: Register fresh Notify

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_{Reviews (1): Last reviewed commit: "feat: implement #651 - feat(cluster-prot..." | Re-trigger Greptile}