[FEATURE][CF-DATAPLANE][TASKS]: Proxy tasks/get, tasks/update, and tasks/cancel

[lucarlig]

Part of #5683.

Blocked by

• [Major-1] Dual-stack stateless core protocol #5679 — per-request client metadata and modern stateless backend lifecycle.
• [Major-4] Multi Round-Trip Requests (MRTR) and required resultType #5682 — MRTR input request/response models and required resultType.
• [Minor-6] Extensions capability field + OpenTelemetry _meta propagation #5689 — extension capability negotiation and advertisement.
• [FEATURE][CF-DATAPLANE][TASKS]: Add stateless backend-scoped task handles #6185 — stateless backend-scoped task handles.

Scope

Implement this in the Rust dataplane repository (contextforge-org/contextforge-data-plane) for MCP 2026-07-28 only.

After a task-capable upstream returns a task handle, the client drives the lifecycle through tasks/get, tasks/update, and tasks/cancel. The dataplane must route each stateless request to the backend that owns the task.

Work

• Implement the RMCP ServerHandler methods for tasks/get, tasks/update, and tasks/cancel.
• Decode the exposed handle from [FEATURE][CF-DATAPLANE][TASKS]: Add stateless backend-scoped task handles #6185, verify that its backend remains authorized and visible in the caller's virtual host, restore the upstream taskId, and forward exactly one upstream RPC.
• Rewrite the taskId in each tasks/get response back to the exposed handle.
• Preserve task status, inputRequests, inputResponses, terminal result/error payloads, metadata, TTL, and changing pollIntervalMs.
• Preserve cooperative, eventually-consistent cancellation and update acknowledgements.
• Let the downstream client control polling and honor pollIntervalMs; do not add a gateway polling loop or blocking tasks/result adapter.
• Ensure a completed deferred tool result is subject to the configured CPEx/plugin policy, or fail closed when that policy cannot be preserved statelessly.
• Ensure upstream Streamable HTTP requests use the restored upstream task ID for Mcp-Name routing.
• Update the MCP routing and method-reference documentation.

Acceptance criteria

• tasks/get routes to the owning backend and returns every valid task state without loss.
• input_required requests round-trip through tasks/update, including partial and eventually-consistent updates.
• tasks/cancel forwards cooperative cancellation and returns a valid empty acknowledgement.
• Malformed, expired, unavailable-backend, cross-virtual-host, and unauthorized handles fail closed without upstream leakage.
• Polling is one downstream request to one upstream request; pollIntervalMs is preserved.
• No MCP session header, in-memory affinity, tasks/list, or tasks/result path is introduced.
• Focused tests cover all statuses, updates, cancellation, headers, authorization, and backend failures.

Out of scope

• Python control-plane implementations and legacy 2025-11-25 Tasks APIs.
• Optional notifications/tasks delivery over subscriptions/listen.