Add an async context data test for `@fedify/express`

[dahlia]

@fedify/express accepts both synchronous and asynchronous context data factories. The async path should be covered so applications can safely load context from sessions or databases.

Add a test in packages/express/src/index.test.ts that verifies integrateFederation() waits for an async contextDataFactory and passes the resolved value to federation.fetch().

Suggested check: mise run check-each express.