Add tests for firstKnock option and KvSpecDeterminer

Add comprehensive test coverage for the firstKnock option functionality:
- Test FederationBuilder handling of firstKnock option with default,
  custom, and explicit values
- Test KvSpecDeterminer class behavior for default specs, custom defaults,
  KV storage/retrieval, and spec overriding
- Ensure proper type safety with KvKey constraints

All tests verify the double-knocking mechanism components work correctly.

Co-Authored-By: Claude <noreply@anthropic.com>