docs: document breaking change requiring SSR environment for Astro component tests

[Ntale3]

Description (required)

This PR documents a breaking change in Astro 6 where rendering .astro components in Vitest tests now requires using an SSR environment (node) instead of client environments (jsdom/happy-dom).

The temporary workaround added in Astro 5.x has been removed to ensure tests run in an environment that matches Astro's SSR build process.

What's included:

• Clear identification of who is affected (Vitest + Container API + client environment users)
• Explanation of what will happen without migration
• Step-by-step migration guide with code examples
• Workspace configuration example for projects needing multiple test environments

Related issues & labels (optional)

• Closes Don’t allow rendering Astro components in Vitest “client” environments astro#14895
• Suggested label: improve documentation

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[ematipico]

Something is wrong with the PR @Ntale3

I believe the problem is that you created your branch from the main branch and then opened the PR towards the v6 branch. Could you fix that?

[delucis]

@ematipico Should be fixed: v6 branch needed updating with latest commits from main and then I pulled those changes in here. So now this PR only shows the relevant changes 👍

[delucis]

Thanks for the PR @Ntale3!

I should have been a bit clearer in my guidance. We should document the change in our upgrade guide: https://github.com/Ntale3/docs/blob/docs/testing-v6-update/src/content/docs/en/guides/upgrade-to/v6.mdx

Our individual docs pages always reflect the current behaviour, and we’ll use the v6 upgrade guide to collect a list of all the changes needed when updating to v6 for people to go through. (Otherwise they would need to hunt all over docs for mentions like this.)

Could you update the PR to make the change to that page instead? You can follow the patterns already in use to see how to structure the content.

[Ntale3]

Thanks for the guidance @delucis let me work as guided