Re-enable middleware exception assertions once Next.js 16.3 reports proxy errors via `onRequestError`

[chargome]

nextjs-16/tests/middleware.test.ts carries a TODO: proxy errors currently not reported via onRequestError with the exception assertions commented out. This should be fixed in Next.js 16.3 — re-enable them once we're on it.

Raised in review of #23924.

[linear-code]

JS-3538

[chargome]

Ok instrumentation is called on the proxy but the error hooks are still not called, tracked in vercel/next.js#83404