Test `onError()` in `@fedify/h3`

[dahlia]

@fedify/h3 depends on onError() to turn a stored Fedify 406 into the final response when H3 reports 404. That small bridge is easy to miss when changing the integration.

Add a test in packages/h3/src/index.test.ts that verifies onError() responds with the stored __fedify_response__ only for 404 errors.

Suggested check: mise run check-each h3.

[Jae-Hyuk-Jang]

I’d like to take on this issue; could you assign it to me?

[dahlia]

@Jae-Hyuk-Jang Assigned!