next.js

019fcca5 - [backport] [test] Fix external rewrite target URL origin (#94829)

[backport] [test] Fix external rewrite target URL origin (#94829) ### Why? Backports https://github.com/vercel/next.js/pull/86863 to `next-15-5` so the skip-trailing-slash-redirect fixture rewrites to the assigned custom domain instead of the protected deployment-specific URL. ### How? - Cherry-picked `551b32d8fc26f801a396391c002eea06ab963c31` with `-x`. - Updates `test/e2e/skip-trailing-slash-redirect/app/middleware.js` to use `https://middleware-external-rewrite-target.vercel.app`. ### Verification - `pnpm build` passed. (`pnpm build-all` is not available on `next-15-5`.) - Not run: `pnpm test-start test/e2e/skip-trailing-slash-redirect/index.test.ts` (stopped per follow-up request to just cherry-pick and open PR). - Not run: `pnpm test-start-turbo test/e2e/skip-trailing-slash-redirect/index.test.ts` (stopped per follow-up request to just cherry-pick and open PR). <!-- NEXT_JS_LLM_PR --> Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>