Add client conformance for SEP-2663 task results

[brixton-guns]

Summary

• add tasks-client-create-handling, where the harness returns a server-directed CreateTaskResult from tools/call and requires the client to retrieve it with tasks/get
• register the scenario in the extensions suite by deriving membership from source.extensionId, and allow extension scenarios to declare the core protocol version they layer on
• update the TypeScript everything-client fixture to negotiate the Tasks extension and drive the polling flow
• add positive and deliberately broken no-poll coverage
• integrate the Tasks extension flow with the suite's per-version wire-schema validation

Why

The server suite already covers SEP-2663, but the client suite could not observe the client requirement to handle either a standard result or CreateTaskResult. This implements the first client scenario proposed in #374.

The scenario is black-box: it proves that the client recognizes the task envelope and follows the returned handle with tasks/get. It does not claim to observe application-level use of the completed result.

Alignment with current main

Rebased onto 49103de, incorporating the 11 upstream commits added since the original PR base.

• layer the Tasks extension on DRAFT_PROTOCOL_VERSION (2026-07-28)
• use the final MISSING_REQUIRED_CLIENT_CAPABILITY code, -32021
• record requests and responses from the custom Tasks mock in the new wire validator
• validate extension-defined CreateTaskResult as a base draft Result envelope, since Tasks v2 types intentionally live outside the core draft schema
• keep wire-schema-valid green for both the conformant and deliberately broken client paths

Validation

• npm run check
• npm run build
• npm test — 488 tests passed
• targeted Tasks, runner, and wire-schema tests — 31 tests passed
• end-to-end conformant client run on Node 22.23.1 — 2/2 checks passed, including wire-schema-valid
• end-to-end no-poll client run — fails only sep-2663-client-handles-polymorphic-result with the expected diagnostic; wire-schema-valid passes

Addresses #374.