Fix stale next data API test URL (#94815)
### Why?
Some app-static prerendered not-found fixtures still fetched
`https://next-data-api.vercel.app/api/random`, which now points at a
different static deployment and returns 404 for the API route. The rest
of the test suite uses `https://next-data-api-endpoint.vercel.app` for
this shared data API.
### How?
Updated the two stale fixture fetches to use
`https://next-data-api-endpoint.vercel.app/api/random`.
### Verification
- `rg "next-data-api\.vercel\.app" test` returned no matches
- `git diff --check` passed for the changed files
- Not run: full e2e suite (URL-only fixture change)
- Not run: prettier/eslint (local `node_modules` is missing, so
`prettier` and `eslint` were not available)
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