fix: serve 404.html also for SSR apps/frameworks

[brillout]

Description

Same logic as #14836.

Someone should run /ecosystem-ci run against this PR.

(I've made two PRs because I'm much less confident Vite's eco CI will be green for this PR.)

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

I don't think we can always handle 404s because in middlewareMode + appType: 'custom' combo, the consumer will be unable to create middlewares after Vite's middlewares with this PR.

[brillout]

AFAIK Vite doesn't have a middleware mode for the preview server.

[bluwy]

Hmm right I forgot this is for preview only.

[bluwy]

We talked about this today. Generally we're pretty indifferent, but since configurePreviewServer allows you to "fix" this today, aligning to the dev behaviour seems like the better option instead. If appType: 'custom' is configured, you would have to handle 404 in dev, so the same should also happen in preview too.

There's more discussed by I'll continue at #14836