[FEATURE][CF-DATAPLANE][TASKS]: Add stateless backend-scoped task handles

[lucarlig]

Part of #5683.

Scope

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

In a virtual host with multiple upstream MCP backends, an upstream-generated taskId is not sufficient to route later tasks/get, tasks/update, and tasks/cancel requests. The dataplane must expose a stable handle without relying on protocol sessions or an in-memory task-to-backend map.

Work

• Add a versioned, opaque, stateless task-handle codec containing the backend route and original upstream taskId.
• Keep the encoding reversible by any dataplane replica; do not depend on Mcp-Session-Id, process memory, or Redis session affinity.
• Reject malformed handles, unsupported handle versions, and backends outside the caller's current virtual-host configuration without leaking backend details.
• Keep the codec usable by both task-creation responses and all tasks/* lifecycle methods.
• Update the MCP routing semantics documentation.

Acceptance criteria

• Arbitrary valid upstream task IDs round-trip without loss.
• Handles from multiple backends cannot collide or route to the wrong backend.
• Malformed, unknown-version, and unavailable-backend handles fail closed with protocol-appropriate errors.
• A handle remains routable after a dataplane restart or on a different replica with the same configuration.
• Focused unit tests cover round trips, invalid input, backend isolation, and versioning.

Out of scope

• Python control-plane task handling.
• Legacy initialize, MCP sessions, SSE, or MCP 2025-11-25 task compatibility.
• Reintroducing removed tasks/list or tasks/result methods.