fix: finish getting Vite 5 working

[benmccann]

vitejs/vite#14818 broke a lot of tests. I mostly fixed it here: 43a745a.

I don't know what's causing the remaining failures, but I think it's also something related to the base path - if I remove the base path from the failing tests then they start working. You can reproduce them with:

cd packages/kit/test/apps/options-2
pnpm build
pnpm test:build

When I run pnpm preview in that directory everything seems to work and it looks like started is added to the body, so I don't know why the tests are failing with a timeout looking for started.

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[teemingc]

The tests are failing because the Vite preview middleware always returns / when making a request to /basepath or /basepath/. This makes it impossible to tell the difference between the two. As a result, the URL getting passed to server.respond is always /basepath/ even when the browser makes a request to /basepath. The server sends a page response instead of a trailing slash redirect, so the browser stays on /basepath while receiving the page for /basepath/

Essentially, the tests get stuck because the browser can't download the scripts while the trailing slash is incorrect and our assets are linked using relative paths. Hence, no started class added to the body element.

Reproduction

1. Run pnpm build && pnpm preview in packages/kit/test/apps/options-2.
2. Navigate to http://localhost:4173/basepath.
3. Open the web inspector console to see the missing asset requests.
4. Check the HTML source to see the hydration script at the bottom of the body. The relative paths to the assets resolve to http://localhost:4173/_app/... as a result of the missing trailing slash.