Test not-found delegation in `@fedify/elysia`

[dahlia]

@fedify/elysia relies on onNotFound to let normal Elysia routes handle non-federation requests. That behavior is part of the integration contract, but it is not covered by a small package-level test.

Add a test in packages/elysia/src/index.test.ts that verifies fedify() does not return a Fedify response when onNotFound is used.

Suggested check: mise run check-each elysia.

[dktsudgg]

I'd like to work on this issue. Could you assign it to me?

[dahlia]

@dktsudgg Assigned. Thanks!

[github-actions]

@dktsudgg This issue has been assigned for over two weeks without updates. Please provide a status update, or unassign yourself if you're unable to continue working on it.

[dktsudgg]

I mistakenly assumed that a pull request for this issue would fail CI because of the duplicate-header bug reported in #970,
so I had been holding off on this work until #970 (pr #972) was resolved.
After re-checking, I confirmed that this work is not affected by #970.