fix: build failure using adapter-cloudflare with paths.base (#13769)

[amodm]

This PR fixes #13769 by ensuring that assets_dir exists before the 404.html is potentially generated.

For details of the issue, please see #13769, but a one line summary is: vite build fails when using @sveltejs/adapter-cloudflare along with a paths.base in svelte.config.js.

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🦋 Changeset detected

Latest commit: fe7290d

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

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[amodm]

The test failure seems unrelated to the changes in this PR. Here's the same test on a different machine:

Running 858 tests using 2 workers
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  239 skipped
  619 passed (1.3m)

> test-options@0.0.1 test:server-side-route-resolution:build **redacted**/kit/packages/kit/test/apps/options
> node -e "fs.rmSync('test/errors.json', { force: true })" && cross-env ROUTER_RESOLUTION=server playwright test


Running 52 tests using 2 workers
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  52 passed (7.8s)