fix: output workers to client assets

[willfarrell]

Clone of #16870, but bases off version-3

Worker chunks from ?worker and ?worker&url imports are emitted to {appDir}/immutable/workers/. In two cases, only the SSR build emits them: when every page has csr: false (the client build is skipped entirely), and when the import lives in a server-only module such as +page.server.js. The files then exist only in output/server/, which adapters never publish, so the worker URL returns a 404 in production.

The build already handles this problem for immutable/assets/ by copying server-emitted files into output/client/. This PR applies the same copy to immutable/workers/ in both build branches. Filenames are content-hashed, so copying over an identical file is harmless, and apps without workers are unaffected.

Tests: the basics app covers the server-only import case by checking that the worker file lands in the client output. The amp app covers the skipped client build case by fetching the worker URL and expecting a 200. Both tests fail without the fix.

Related: #12438. That issue also stems from worker files being overlooked, but it concerns the $service-worker build list and is not resolved here.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from a6a1364:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/a6a1364698428312393810f18adc5f714ba2b5a3

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16929

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

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Latest commit: a6a1364

The changes in this PR will be included in the next version bump.

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[Nic-Polumeyv]

Awesome!

[teemingc]

Thanks! I separated the no-csr tests a little bit to make it clearer when a test fails