[TESTING][CF-DATAPLANE][TASKS]: Add modern Tasks extension conformance coverage

[lucarlig]

Part of #5683.

Blocked by

• [Major-1] Dual-stack stateless core protocol #5679 — modern stateless request lifecycle.
• [Major-4] Multi Round-Trip Requests (MRTR) and required resultType #5682 — required resultType and MRTR behavior.
• [Minor-6] Extensions capability field + OpenTelemetry _meta propagation #5689 — extension capability negotiation.
• [FEATURE][CF-DATAPLANE][TASKS]: Add stateless backend-scoped task handles #6185 — task-handle codec.
• [FEATURE][CF-DATAPLANE][TASKS]: Forward server-directed task results from tools/call #6187 — task-augmented tools/call forwarding.
• [FEATURE][CF-DATAPLANE][TASKS]: Proxy tasks/get, tasks/update, and tasks/cancel #6186 — tasks/get, tasks/update, and tasks/cancel proxying.

Scope

Add Rust dataplane tests and fixtures for the MCP 2026-07-28 io.modelcontextprotocol/tasks extension. This is distinct from the closed MCP 2025-11-25 Python/control-plane coverage in #4295.

Work

• Add modern RMCP task-capable backend fixtures, including a multi-backend virtual host.
• Cover capability-on and capability-off requests using per-request client metadata.
• Exercise server-directed CreateTaskResult, tasks/get polling, input_required plus tasks/update, cooperative tasks/cancel, and completed/failed/cancelled terminal states.
• Verify task-handle rewriting and routing across replicas/restarts without MCP sessions.
• Verify downstream and upstream Mcp-Method / Mcp-Name values use the exposed and restored task IDs at the correct boundary.
• Verify removed legacy methods are not exposed.
• Run the upstream Tasks extension conformance scenarios that are applicable to a federating proxy, or document any fixture limitation.
• Add regression coverage for authorization, plugin-policy, malformed-handle, and partial-backend-failure cases.

Acceptance criteria

• A task-capable modern client completes a full tool → task → poll lifecycle through the dataplane.
• A client without io.modelcontextprotocol/tasks never receives a task handle.
• Working, input-required, completed, failed, and cancelled states are covered.
• Multi-backend task IDs cannot collide or cross virtual-host/authorization boundaries.
• Polling survives dataplane restart/replica changes with no Mcp-Session-Id.
• tasks/list and tasks/result return method-not-found on the modern dataplane path.
• Applicable official Tasks extension conformance scenarios pass.

Out of scope

• Python control-plane tests.
• Legacy initialize, session, SSE, or MCP 2025-11-25 compatibility tests.
• Optional task-status notifications unless a separate scoped issue enables them.