fix: prevent vitest from hanging

[teemingc]

fixes https://github.com/sveltejs/kit/issues/12305 closes https://github.com/sveltejs/kit/pull/12306

This is an alternative to https://github.com/sveltejs/kit/pull/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 instead of much earlier when adapter.adapt is invoked.

We could still use the newly added test from https://github.com/sveltejs/kit/pull/12306

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• [x] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• [x] This message body should clearly illustrate what problems it solves.
• [ ] Ideally, include a test that fails without this PR but passes with it.

Tests

• [ ] Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• [x] If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• [x] Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 90b0a866b96debd58928e3f46507ea56d5402f9a

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Rich-Harris]

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

this is an automated message