The integration project now aliases server-only to a tiny empty stub
so tests can import modules that carry 'import "server-only"' without
the throwing real implementation firing during prepare. Unit tests do
NOT get the alias — a pure module with a server import is exactly the
kind of leak the unit project's missing condition is meant to surface.
Vitest config: integration project now resolves with the 'react-server'
condition, so server-only modules can be imported directly from tests.
The unit project deliberately does NOT set it: a unit test pulling in a
server-only module must fail loudly, because a pure module with a server
import is exactly the kind of leak that should not reach production.
