Test response forwarding in `@fedify/elysia`

[dahlia]

@fedify/elysia should preserve the response produced by Fedify when Fedify handles a request. A regression here would make valid federation responses look wrong to Elysia users.

Add a test in packages/elysia/src/index.test.ts that verifies status, headers, and body from federation.fetch() are forwarded by fedify().

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!

[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]

while working on this issue, I discovered #970, and the response-forwarding test requested here is directly affected by it.
every response header gets duplicated, so a strict header assertion fails. I verified this by running the test against the current main.

I will open a pull request for this issue once the fix (PR #972) is merged and forward-merged into main.