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