fetchJson: fetch with proper error handling

Implement fetchJson(fetch, url)

The single most common mistake in real API code: assuming fetch rejects on an HTTP error. It doesn't — fetch only rejects on a network failure. A 404 or 500 still resolves, with response.ok === false. Forgetting to check leads to trying to .json() an error page.

Write fetchJson(fetch, url) that:

• Calls fetch(url).
• If response.ok is false, throws an error whose message includes the status code (e.g. "Request failed with status 404").
• Otherwise, returns the parsed JSON body.

const data = await fetchJson(fetch, "/api/users/1"); // { id: 1, name: "Ada" }
await fetchJson(fetch, "/api/missing"); // throws "Request failed with status 404"

fetch is passed in as an argument here (rather than using the global) so it's easy to test — that's also a genuinely good pattern for testable API code.