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

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.

Validation

• npm run check
• npm run build
• npm test — 423 tests passed
• end-to-end conformant client run — new check passes
• end-to-end no-poll client run — new check fails with the expected diagnostic
• official TypeScript SDK main (f60dff06), using a temporary scenario adapter — the new check correctly fails when the SDK rejects resultType: "task" and does not issue tasks/get

Addresses #374.