chore: test server directly

[Rich-Harris]

Companion to #16872. I wanted to see if we can make direct calls to server.respond with no HTTP intermediary. For the most part it looks like we can, and in fact by doing so I learned that we respond to HEAD requests incorrectly by including a body (probably immaterial, since the body is likely discarded by any layer between the user and the server, including fetch itself, but still interesting to note).

Note that it requires us to explicitly pass accept: '*/*' (which fetch implies) and handle redirects ourselves.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 4bea774:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/4bea7740859d2cf9d0e77ae5e51bc4e88f4bc24d

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16879

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4bea774

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16879-svelte.vercel.app/

[Nic-Polumeyv]

Yeah. This is good. I'll go through this properly tomorrow.

[Nic-Polumeyv]

The unit CI rows run vitest alone, so the globalSetup build is the only thing that creates .svelte-kit/output there. The double build only happens locally when running both suites back to back.

Stripping in server.respond also covers page routes, and leaves event.fetch alone, which never went through the HTTP layer in prod anyway.

[Nic-Polumeyv]

Went through the rest of the suite for the same class. The only other candidate is request body can be read slow, where the body is an in-memory buffer under a direct Request, so nothing is slow anymore and the QUERY test covers the unconsumed-body delivery. Left it in. Everything else traces to kit code, including the platform-sounding ones (the Vite trailing-slash redirect and static-serving tests pin kit's preview middleware and allow-list).

[Nic-Polumeyv]

Folded into #16872 and rebased.