fix(css): read the live root in the OnceExit url rewriter (fix #23348)

[lazerg]

Vite 8.2.2 moved the built-in CSS url rewriter from a Once visitor to OnceExit (#22983), so it can see content injected by other plugins' own OnceExit hooks. That works fine for plugins that mutate the existing root in place, but breaks for plugins that reassign result.root outright, which is what postcss-lightningcss does.

PostCSS's OnceExit dispatch loop captures root once, before any OnceExit hook runs, and passes that same reference to every plugin registered for the event. If an earlier plugin sets result.root to a brand new tree instead of mutating the node it was handed, later OnceExit hooks still get the stale, pre-reassignment root as their argument, even though result.root has already moved on. Our rewriter is registered last so it runs after plugins like postcss-lightningcss, walks that stale root, and rewrites urls on a tree that never makes it into the final output.

This reads result.root instead of trusting the root argument, since helpers.result is a live reference. postcss-modules still mutates the tree it's given, so the #22983 fix keeps working, and plugins that swap the whole tree are now covered too. Added a regression test modeled on a plugin that replaces result.root, alongside the existing OnceExit-injection test.

Fixes #23348

[github-actions]

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[lazerg]

🤦🏻

[lazerg]

Hi @sapphi-red, replying to the bot flag above since it asks for that if it's wrong.

I'm a real person, not a bot account. This PR fixes a genuine regression in the OnceExit CSS url rewriter from #22983, with a repro and a test. Happy to answer any questions about the change if that helps confirm it. Could this get reopened?

[sapphi-red]

I tried to re-open, but failed because the repo is deleted.