fix: make server assets accessible during vite preview

[eltigerchino]

fixes #9961

This moves the server assets copying to client assets closer to the vite build process instead of as part of the adapter process. This ensures server assets are always included in .svelte-kit/output/client so that vite preview serves them correctly.

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Latest commit: 74a2183

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

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@sveltejs/kit	Patch

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[ghostdevv]

The history of this code is super weird it seems to have been changed/reverted multiple times - looks like ben has been editing this part the most so requesting a review from him