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

[tomdps]

Closes #651

[greptile-apps]

Greptile Summary

This PR makes WebSocket passthrough cancellation registration deterministic and adds cancellation lifecycle regression coverage.

• Introduces a readiness gate so passthrough work starts only after its abort handle is registered.
• Adds a drop guard to release request IDs and cancellation entries on completion, cancellation, or panic.
• Adds unit and integration tests for fast completion, cancellation cleanup, and request-ID reuse.

Confidence Score: 3/5

This PR should not merge until cleanup for an old request is prevented from deleting the cancellation handle of a newer request with the same ID.

The separate admission and cancellation registry removals create an ownership race in which request-ID reuse can leave an active retry without a usable cancellation handle.

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

Important Files Changed

Filename	Overview
crates/openengine-cluster-server/src/websocket.rs	Adds gated task startup and unconditional cleanup, but cleanup can remove a newer same-ID request's cancellation handle.
crates/openengine-cluster-server/src/websocket/tests.rs	Adds deterministic unit coverage for the original spawn-before-registration stale-entry race.
crates/openengine-cluster-server/tests/websocket.rs	Adds integration coverage for cancellation cleanup, ID reuse, and rapid completion behavior, but not the cross-task cleanup race.

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

[greptile-apps]

Cleanup removes the retry handle

When a cancelled or completed request is immediately retried with the same ID, the old guard releases in_flight_ids before separately removing from cancel_registry, allowing the retry's newly registered abort handle to be removed and causing a later $/cancelRequest to be silently ignored.

[tomdps]

Post-validation review found an additional P1 ownership race: the old request guard releases in_flight_ids and then removes cancel_registry separately, so an immediate same-ID retry can register between those operations and have its new cancellation handle removed by the old guard. This pull request was removed from the merge queue and is superseded by a fresh #651 hardening run whose contract requires generation/ownership-safe cleanup.