fix: prevent incorrect trailing slash redirect for prerendered root page when paths.base is set

[teemingc]

fixes #9982

This PR reinforces the behaviour of always redirecting /base to /base/ in #9343 by:

• fixing the client-side trailing slash redirect incorrectly redirecting from /base/ to /base (because no trailing slash option is set).
• when running vite preview and visiting the prerendered root page without the trailing slash /base, correctly redirect to the trailing slash path /base/.
• keep search params intact for vite preview trailing slash redirect

Debating adding a test for this because it would require setting up another test project just to test the behaviour of prerendering a root page with paths.base set.

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[benmccann]

Can we add an explanation of why that is? It seems quite surprising to me that we would ignore the trailing slash option for the root page when base is set. I remember putting a lot of effort into an earlier version of Vite to allow it to work without a trailing slash and I don't understand why we'd have this new behavior

[teemingc]

The original explanation is in this previous PR #9343 However it was not applied in vite preview, hence this PR. But I think this got removed in 2.0 because vite always does it? I have to check

[benmccann]

Oops, sorry, I didn't mean to comment on a closed PR. I was on mobile and thought I was looking at #11357