fix!: only serve dev static assets from publish dir as fallback

[Copilot]

Fixes #436

Problem

When using @netlify/vite-plugin, vite dev (or e.g. astro dev) was incorrectly serving files from the Netlify publish dir (e.g. dist/) instead of the source directory during vite dev.

This causes various problems, the most basic of which is that if you've ever previously run vite build (or e.g. `astro build) your dev server scans the production build output dir and serves assets from it. This is confusing and incorrect.

Solution

I can't think of any reason to ever augment the directories explicitly passed to @netlify/dev's options with a default directory. In every known use case (@netlify/vite-plugin, @netlify/nuxt...), we pass the known, exhaustive set of directories that should be used to serve static assets in dev, so we should respect these as is.

It seems reasonable to use some sensible defaults here when not provided (for hypothetical future or direct use cases).

This PR makes this change.

Original prompt

Issue Title: Using @vite/netlify-plugin causes Vite to serve dist instead of usual vite dev behaviour
Issue Description: Hi, I've noticed a very odd issue with your plugin. It appears @netlify/vite-plugin alters Vite's regular vite dev behaviour from serving transformed source to instead serving the dist folder, just like vite preview.

Here's a reproduction:
https://github.com/alextompkins/netlify-vite-plugin-serve-dist-issue
Fixes https://linear.app/netlify/issue/FRB-1969/using-vitenetlify-plugin-causes-vite-to-serve-dist-instead-of-usual

Comment by User db2967f5-c49c-428b-ae14-6c61855027c1:
netlify/primitives

Comment by User c8ba5c18-8b2b-41c2-8484-9ca4ad793475:
📋 I wasn't able to determine which GitHub repository to work in.

Please tell me which repository this work should be done in using the format owner/repository-name (e.g., github/copilot), then I'll automatically start working on it!

Comment by User c8ba5c18-8b2b-41c2-8484-9ca4ad793475:
Waiting for https://linear.app/netlify/profiles/serhalp to link their GitHub account. Click to authorize →

Comment by User :
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Comment by User :
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[serhalp]

@copilot what does this have to do with SPAs and redirects? did you cross wires with another issue?

also, can you compare this with the behaviour implemented in https://github.com/netlify/cli/? when does it use static files in the publish dir during dev?

[Copilot]

@copilot what does this have to do with SPAs and redirects? did you cross wires with another issue?

also, can you compare this with the behaviour implemented in https://github.com/netlify/cli/? whe...

The SPA redirect is directly related to the issue. Here's how:

1. The reproduction has a SPA-style rewrite rule: /* -> /index.html (status 200)
2. When a request comes in, the redirect handler matches it and rewrites to /index.html
3. The redirect handler then looks for /index.html in the static directories
4. With the old code, static directories were [dist, projectRoot, publicDir], so dist/index.html was found first
5. The middleware sees result.type === 'redirect' (not 'static'), so it serves the response directly (line 117-120 in main.ts)
6. This causes the built dist/index.html to be served instead of the source index.html transformed by Vite

Without the SPA redirect, the issue would still exist but be less obvious - direct requests for files that exist in both dist and source would serve from dist.

Regarding comparison with Netlify CLI: The CLI doesn't integrate with Vite's dev server, so it doesn't have this conflict. During vite dev, we explicitly want to serve from source directories only (projectRoot + publicDir), not from the build output. The fix ensures that when vite-plugin provides custom directories, those are used exclusively. When @netlify/dev is used standalone (no custom directories), it still falls back to the original behavior of using build.publish.

Commit: 71ed852