Dev server dies with MISSING_EXPORT when server code pulls in file-type; serverExternalPackages not honored by rsc/ssr dep optimizers

**Context**

An app whose server code imports `file-type` — the common case is Payload 3, whose `payload/dist/uploads/*` imports `{ fileTypeFromFile } from 'file-type'`. Running `vinext dev` on the Cloudflare runtime.

**Observed**

The dependency optimizer resolves `file-type/core.js` and fails named-export interop with three `MISSING_EXPORT` errors (`fileTypeFromFile` etc.); the dev server dies. Two separate gaps:

1. `file-type`'s dual CJS/ESM shape breaks Rolldown's pre-bundle interop, but nothing excluded it — the runtime resolvers load it fine.
2. Packages listed in `serverExternalPackages` are excluded from the **client** optimizer but not from the **rsc/ssr** optimizers, even though those environments already load them through the runtime resolver. Their node-only conditional exports resolve to the wrong entry inside the optimizer pipeline.

**Workaround (app-side vite.config)**

```ts
export default defineConfig({
optimizeDeps: { exclude: ["file-type"] },
ssr: { external: ["file-type"] },
});
```

**Expected**

Both classes of package should stay out of every pre-bundle without per-app workarounds.

Fixed by #3235.

Start with the rsc/ssr dependency optimizers and their handling of serverExternalPackages, comparing them with the client optimizer and runtime resolver described here. Use #3235 as the implementation reference, then verify that file-type and packages listed in serverExternalPackages stay out of every pre-bundle without app-side optimizeDeps or ssr workarounds.

Written by the indexing model from the issue text.