SvelteKitAuth not working with SvelteKit's base path

Environment

  System:
    OS: Windows 10 10.0.22621
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
    Memory: 6.64 GB / 15.41 GB
  Binaries:
    Node: 18.17.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.19 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 9.6.7 - C:\Program Files\nodejs\npm.CMD
    pnpm: 8.5.1 - ~\AppData\Local\pnpm\pnpm.CMD
  Browsers:
    Edge: Spartan (44.22621.2283.0), Chromium (117.0.2045.36)
    Internet Explorer: 11.0.22621.1

Reproduction URL

https://github.com/aakash14goplani/SvelteKit-Auth0-NextAuth/tree/basepath and https://sveltekit-basepath.vercel.app/hello/

Describe the issue

Unable to initiate authentication flow with SvelteKitAuth while using base path.

This issue was recently claimed to be fixed via https://github.com/nextauthjs/next-auth/pull/8231 and https://github.com/nextauthjs/next-auth/issues/8230. Even after following instructions specified within those fix, authentication flow is broken.

How to reproduce

Add base path in SvelteKit
```
kit: {
 paths: {
   base: '/hello'
 }
}
```

Update SvelteKitAuth configuration object

const config: SvelteKitAuthConfig = {
  providers: [...],
  prefix: '/hello',
  ...
}

Initiate sign-in process and observe the error :

NOTE: I also tried experimenting with _NEXTAUTHURL by setting its value to -> http://localhost:4200, http://localhost:4200/hello and http://localhost:4200/hello/auth but none of them worked for me!

Dependency Versions Used:

"dependencies": {
  "@auth/core": "^0.15.1",
  "@auth/sveltekit": "^0.3.7"
},
"devDependencies": {
  "@sveltejs/adapter-vercel": "^3.0.3",
  "@sveltejs/kit": "^1.25.0",
  "svelte": "^4.2.1",
  "svelte-preprocess": "^5.0.4",
  "typescript": "^5.2.2",
  "vite": "^4.4.9"
}

Expected behavior

The authentication flow should work with the base path.

We could not detect a valid reproduction link. Make sure to follow the bug report template carefully.

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nextauthjs / next-auth