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

[tomdps]

Closes #651

[greptile-apps]

Greptile Summary

Adds a WebSocket data-plane binding while sharing protocol dispatch and subscription machinery with NDJSON.

• Introduces WebSocket client and server transports with multiplexed unary requests and subscriptions.
• Adds $/cancelRequest, shared dispatch/admission infrastructure, and transport-neutral subscription clients.
• Extends protocol artifacts, documentation, fixtures, and cross-transport integration tests.

Confidence Score: 3/5

This PR should not merge until WebSocket cancellation reliably removes aborted requests from the connection's in-flight bookkeeping.

Cancelling a pending passthrough request bypasses normal ID cleanup, leaving the ID classified as in flight, while the task-registration race also retains completed abort handles on long-lived connections.

Files Needing Attention: crates/openengine-cluster-server/src/websocket.rs

Security Review

The WebSocket cancellation path can retain cancelled request IDs for the connection lifetime, enabling stale-state growth and preventing those IDs from being reused.

Important Files Changed

Filename	Overview
crates/openengine-cluster-server/src/websocket.rs	Adds WebSocket serving and cancellation, but task lifecycle races retain in-flight IDs after cancellation and completed abort handles after fast dispatch.
crates/openengine-cluster-client/src/websocket.rs	Adds the client WebSocket frame sink and response pump using the shared multiplexing implementation.
crates/openengine-cluster-client/src/multiplex.rs	Centralizes request correlation, subscription routing, cancellation serialization, and transport lifecycle.
crates/openengine-cluster-server/src/stdio/dispatch.rs	Extracts shared admission, dispatch, subscription spawning, and connection shutdown behavior without an identified regression.
crates/openengine-cluster-protocol/src/watch.rs	Adds the typed CancelRequestParams payload used by the new cancellation notification.
protocol/openengine-cluster/v1/schema.json	Updates the generated schema with the additive cancellation notification contract.

Sequence Diagram

sequenceDiagram
    participant C as WebSocket client
    participant S as WebSocket server
    participant B as Backend
    C->>S: "Request id=1"
    S->>S: "Insert id=1 into in_flight_ids"
    S->>B: dispatch(request)
    C->>S: "$/cancelRequest id=1"
    S->>S: Remove AbortHandle and abort task
    Note over S: id=1 remains in in_flight_ids
    C->>S: "New request id=1"
    S-->>C: DUPLICATE_REQUEST_ID

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

[greptile-apps]

Cancellation retains in-flight IDs

When a client cancels a passthrough request while backend dispatch is pending, aborting the task bypasses the cleanup that removes its ID from in_flight_ids, causing later requests that reuse the ID to be rejected as duplicates and cancelled IDs to accumulate until disconnection.

[greptile-apps]

Completed tasks leave abort handles

The task can complete and remove its registry entry before this subsequent insertion, leaving an already-completed abort handle stored for the rest of the connection; fast requests therefore accumulate stale cancellation entries on long-lived connections.