fix: prevent vitest from hanging

[eltigerchino]

fixes #12305
closes #12306

This is an alternative to #12306 . Instead of creating the emulator every time the Vite dev and preview middlewares are invoked, we await Cloudflare's platform proxy only when the adapter emulated platform is accessed, rather than much earlier when adapter.adapt is invoked.

We could still use the newly added test from #12306

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-12830-svelte.vercel.app/

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