fix(adapter-vercel): make normalization redirects root-relative

[teemingc]

What this does

Converts SvelteKit normalization redirects to root-relative Location headers in the Vercel serverless wrapper. This prevents Vercel from resolving relative normalization redirects against its internal ISR rewrite URL.

Stacked on #16903, which adds the failing ISR + trailingSlash: 'always' test.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 82d7be7:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/82d7be77c6f3ba0268edac9c1b98cc54fab8e3e9

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16905

[changeset-bot]

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Latest commit: 82d7be7

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16905-svelte.vercel.app/

[Nic-Polumeyv]

The problem is upstream of the location header: __pathname never contains the trailing slash, so kit thinks the visitor is at /isr-trailing-slash-absolute even when they requested /isr-trailing-slash-absolute/, and redirects them to where they already are. That loops no matter what the location header looks like. If the route's src captures the trailing slash and passes it through (__pathname=/isr-trailing-slash-absolute$1), kit sees the real path and doesn't redirect at all.