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

[dahlia]

@fedify/fresh uses Fresh ctx.next() when Fedify does not handle a request. This is the basic coexistence path for Fresh pages and federation endpoints.

Add a test in packages/fresh/src/mod.test.ts that verifies integrateFetchOptions() wires onNotFound to ctx.next().

Suggested check: mise run check-each fresh.

[rewrite0w0]

Hi! I'd like to work on this test as well as #863 and #864/#865. Could you please assign it to me?

[sij411]

@rewrite0w0 Assigned. Thanks.