Set `build.ssrEmitAssets: true` so the SSR environment writes asset
files (CSS, etc.) that JS chunks reference. Vite defaults
`emitAssets` to `false` for environments with `consumer: "server"`,
so when the CSS plugin rewrites a server-component CSS import into
`import "<hash>.css"` and emits the CSS asset, the asset file is
subsequently stripped from the bundle by Vite's asset-cleanup hook.
The dangling import then crashes `vinext start` with
ERR_MODULE_NOT_FOUND on `dist/server/ssr/style.css`.
We set the top-level `build.ssrEmitAssets` rather than
`environments.ssr.build.emitAssets` because Vite re-applies the
top-level value during `resolveConfig` and would otherwise overwrite
any per-environment value. Mirrors `@vitejs/plugin-rsc`'s existing
`emitAssets: true` on the `rsc` environment.
Adds tests asserting both the App Router and Pages Router SSR
environments resolve to `emitAssets: true` and that every CSS import
in the SSR build output resolves to a real file on disk.
James Anderson