### What?
Fixes#58570
### How?
Include the **basePath** to the **fetchUrl** to ensure the relative URL
matches the app when deployed under a **basePath**.
### Tested?
I have added an **e2e** test with a basic custom server & server action
redirect.
This test was confirmed to **catch** the bug when running it without the
fix in place. When running it you will get the failed result.
```
FAIL test/e2e/app-dir/app-basepath-custom-server/index.test.ts (12.293 s)
custom-app-render
✕ redirects properly when server action handler uses `redirect` (1661 ms)
● custom-app-render › redirects properly when server action handler uses `redirect`
expect(received).not.toEqual(expected) // deep equality
Expected: not ["/base/another", 200]
45 | // if broken, this will include a 200 from the /base/another indicating a full page redirect
46 | responses.forEach((res) => {
> 47 | expect(res).not.toEqual(['/base/another', 200])
| ^
48 | })
49 | })
50 | }
at toEqual (e2e/app-dir/app-basepath-custom-server/index.test.ts:47:25)
at Array.forEach (<anonymous>)
at Object.forEach (e2e/app-dir/app-basepath-custom-server/index.test.ts:46:17)
Test Suites: 1 failed, 1 total
Tests: 1 failed, 1 total
Snapshots: 0 total
Time: 12.321 s, estimated 22 s
Ran all test suites matching /test\/e2e\/app-dir\/app-basepath-custom-server/i.
ELIFECYCLE Command failed with exit code 1.
ELIFECYCLE Command failed with exit code 1.
ELIFECYCLE Command failed with exit code 1.
```
### Notes
Not sure if there are any edge cases where the `fetchUrl` is now broken
in other use cases where there is no **basePath**, I assume the string
would be empty `""` and result in the same URL as before, but not sure?
### Disclosure
~I am not that familiar with the Next.js code base and this is my first
PR. I was struggling to find out how to grab the **basePath** from
`next.config.js`, but then noticed the **assetPrefix** inside the
function matched, so decided to use that for minimal change. I don't
know if there are any caveats with this approach, but could consider
switching to pull directly from the config file, if that's possible?~
**Update:** Figured out where the **basePath** came from and switched
it.
---------
Co-authored-by: Colton Ehrman <cehrman@paypal.com>
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
