fix: prevent vitest from hanging when using cloudflare/cloudflare-workers

[wooseopkim]

Adapter.emulate is a property used for adapter-cloudflare and adapter-cloudflare-workers. Because it is invoked earlier than needed, vitest runs hang as the Miniflare instance is not released. This PR makes Adapter.emulator called right before its return value is needed. prerender code is changed too, but it seems that adapter-cloudflare/cloudflare-workers don't allow pages to be prerendered. Return type of dev was changed because of ESLint error banning async functions without any awaits. It still returns an allowed type.

To make sure dev/preview server works with no problems, I've visually checked the rendered output of packages/kit/test/apps/cloudflare by running dev and build/preview. Also I've added a Playwright test to make sure the page is properly rendered and able to connect to Cloudflare KV namespaces.

Fixes #12305.

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[benmccann]

this comment is confusing. it seems to be saying that the teardownTimeout option is added to workaround issue #12305. however, that issue would be closed by this PR. can you add some text to go with the link and give some context?

[wooseopkim]

@benmccann Actually this is not necessary at all. It has the default value of 10000 or 10 seconds. I didn't want to make the test take 10 seconds longer, which happens when this fix goes broken and the test case fails. Do you think I can remove this entirely, or should I write in detail why I specified smaller value than the default in this case?

[wooseopkim]

I took the former approach, i.e. removing the timeout field.

[wooseopkim]

@benmccann Hi, here's a friendly reminder about this PR.

[dummdidumm]

Thank you for the PR - we went with #12830 instead, so closing this one.