chore: run the basics server tests against Server.respond

[Nic-Polumeyv]

The server tests in basics never needed a browser: they assert on status, headers and body. Running them through Playwright with JavaScript off meant a preview server and a browser per project, repeated in every no-js leg of the matrix, and a 1 to 2 minute loop to run one of them locally. The file said so itself (server.test.js:33, "this would ideally be a unit test of Server").

They now run under vitest from unit-test/server.spec.js, calling the built app's Server.respond directly with no HTTP in between. With KIT_TEST_DEV set the same file goes through Vite's dev server in middleware mode instead, so both modes keep their coverage. The suite runs from the app's own test:dev and test:build after Playwright, against the build Playwright already made, so nothing builds twice and the unit job is untouched. 13 tests stay in Playwright because they assert what vite preview and the dev server do around Server (static assets, prerendered output).

Stacked on #17036: three of the moved HEAD tests need it in build mode.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 877bcff:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/877bcff3d812590929de85fcb94c593bbac08acd

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16872

[changeset-bot]

⚠️ No Changeset found

Latest commit: 877bcff

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16872-svelte.vercel.app/

[Rich-Harris]

Noodling on some tweaks in #16879. One thing neither PR covers AFAICT: the existing setup tests the dev server as well as the prod build, but this only tests the latter IIUC?

[Nic-Polumeyv]

Dev is back on 535985a, a second vitest project runs the same spec through Vite in middleware mode. 79 build / 67 dev, 12 skipped where Playwright skipped them too.