[Schema] SEP-2663 Tasks: Task shape, resultType discriminator, tasks/* request/result types

Schema portion of SEP-2663 (Tasks extension, io.modelcontextprotocol/tasks) for the 2026-07-28 release. Tracked by umbrella #335; sits on the SEP-2133 extensions framework (#344).

Scope

• Mcp\Schema\Task: taskId, status, createdAt, lastUpdatedAt, ttlMs, pollIntervalMs, statusMessage, result, error, inputRequests. Flat on the wire in both places it appears (no nested task key). Enforces the status semantics: a tool that ran and reported a problem is completed with isError; failed is reserved for protocol-level errors and carries the error inlined.
• Mcp\Schema\Enum\TaskStatus: working, input_required, completed, failed, cancelled (what the merged spec and conformance suite use — not pending/running).
• Mcp\Schema\Enum\ResultType::Task ("task") alongside Complete and InputRequired.
• CreateTaskResult — a result in its own right (resultType: "task" + the task envelope), returned by any task-augmentable handler instead of a discriminator bolted onto CallToolResult.
• TaskResult for tasks/get; tasks/update and tasks/cancel answer EmptyResult.
• TasksGetRequest, TasksUpdateRequest (taskId, inputResponses), TasksCancelRequest.
• inputRequests reuse the multi round-trip (SEP-2322) encoding: a map of server-assigned keys to bare method/params pairs.

Notes

SEP-2663 supersedes SEP-1686 (#139, closed).

Start with the existing schema and enum patterns for TaskStatus, ResultType, CallToolResult, and EmptyResult, then review the extensions framework in #344 and the umbrella scope in #335. Implement the listed task envelope, result, request, and enum types, preserving the flat wire shape and inputRequests encoding; done means all named types and status semantics match the merged spec and conformance expectations.