Test not-acceptable fallback in `@fedify/fresh`

[dahlia]

@fedify/fresh should let Fresh render HTML first, then return 406 only when Fresh also returns 404. A focused test would protect that content negotiation behavior.

Add a test in packages/fresh/src/mod.test.ts that verifies onNotAcceptable returns the Fresh response for non-404 results and returns a 406 response for 404.

Suggested check: mise run check-each fresh.

[rewrite0w0]

Hi! Could you please assign this issue to me?!

[sij411]

@rewrite0w0 Assigned. Thanks.