Middleware URL normalization does not respect `skipTrailingSlashRedirect`

### Link to the code that reproduces this issue https://codesandbox.io/p/devbox/vigorous-wilson-25ycfg ### To Reproduce 1. Start the application 2. Click the 2 links 3. Observe the console output ### Current vs. Expected behavior Current: always returns pathname `/foo` Expected: returns `/foo` or `/foo/` depending on the link ### Provide environment information ```bash Operating System: Platform: linux Arch: x64 Version: #1 SMP PREEMPT_DYNAMIC Sun Aug 6 20:05:33 UTC 2023 Available memory (MB): 4102 Available CPU cores: 2 Binaries: Node: 20.9.0 npm: 9.8.1 Yarn: 1.22.19 pnpm: 8.10.2 Relevant Packages: next: 15.0.0-canary.25 // Latest available version is detected (15.0.0-canary.25). eslint-config-next: N/A react: 19.0.0-beta-04b058868c-20240508 react-dom: 19.0.0-beta-04b058868c-20240508 typescript: 5.1.3 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Middleware, Pages Router ### Which stage(s) are affected? (Select all that apply) next dev (local), next build (local), next start (local), Vercel (Deployed), Other (Deployed) ### Additional context The issue turns up in conjunction with `config.skipTrailingSlashRedirect` being set to `true`. The issue leads to all client-side navigation locking up in an infinite redirect loop when using middleware to redirect to trailing slash variants of the same url, e.g. from `/foo` to `/foo/`. In the middleware the pathname is still `/foo` even after the redirect, which starts the loop. The issue (most likely) stems from the URL normalization that Next.js applies to `/_next/data/*` urls. The respective `.json` URLs are always normalized to whatever `config.trailingSlash` is set to, disregarding the setting of `config.skipTrailingSlashRedirect`. This seems related, but not identical, to #66738. The source of the error could be close to the source of #58320 as well. From the looks of it, this doesn't affect the app router. Here the `pathname` looked correct in both…