[chore] Fix headless docs build

[slax57]

Problem

#11186 bumped zod to v4 in the monorepo, and although astro should still be using zod v3, we hit that issue where astro still uses the globally installed version of zod instead of the scoped one when building the website.

This makes our CI fail: https://github.com/marmelab/react-admin/actions/runs/22909235564/job/66476585567

Solution

Lucky for us there is a simple workaround:

Add the following to adtro config:

import { defineConfig } from 'astro/config';
export default defineConfig({
	vite: {
		ssr: {
			noExternal: ['zod'],
		},
	},
});

How To Test

To reproduce the CI issue:

make install
cd docs_headless
yarn build

This should fail on master and be fixed with this PR.

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.

[slax57]

Any yarn command I run keep changing this file, so I figured I might as well commit the 'fixed' version.