Add basic KV tests for `@fedify/cfworkers`

[dahlia]

@fedify/cfworkers provides a Workers KV adapter, but the package lacks focused unit tests for basic KV operations. A small in-memory mock would make this adapter safer to change.

Add tests in packages/cfworkers/src/mod.test.ts that cover WorkersKvStore.get(), WorkersKvStore.set(), and WorkersKvStore.delete() with a minimal KV namespace mock.

Suggested check: mise run check-each cfworkers.