fix(validation): accept extension result envelopes

[jstar0]

Summary

• Treat open resultType discriminators such as SEP-2663 task as extension results.
• Validate extension results against the generic JSON-RPC result envelope until a versioned extension schema is available.
• Add regression coverage for a task result returned from tools/call.

Verification

• npm run build
• npm test (44 test files, 525 tests passed)
• npm run check (typecheck, ESLint, and Prettier)
• Pre-push hook passed (format check and full test suite)

Fixes #424

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/@modelcontextprotocol/conformance@468

commit: 49bea9e

[pcarleton]

Thanks @jstar0. I pushed one small commit on top (49bea9e): only known extension resultTypes (today just "task") take the envelope-only path; any other unrecognised value is still validated as the method's result, with the value named in the failure.

@panyam @LucaButBoring FYI. Follow-ups I'd like your read on: whether "task" is the only value to allow-list for now, and making resultType a proper discriminator in the core schema (we likely need new a spec issue). #473's typed CreateTaskResult validation is something we can consider next.