fix(onboard): serve embedded.js through Nuxt's Vite dev handler

[antfubot]

Summary

When @nuxt/devtools is not installed, nuxt dev answers /__devtools/embedded.js with the app's HTML page (content-type: text/html). The browser then rejects the module script, so the onboarding button never shows up. This affects @nuxt/devtools-onboard@4.0.0-beta.6.

The onboarding middleware was mounted at the root of the client Vite server. Nuxt's Vite dev handler only passes a request to Vite when some middleware is mounted on a path longer than /. Otherwise it rewrites the URL to /__skip_vite/..., the middleware's base check fails, and the request falls through to the Nuxt renderer.

The middleware is now mounted on /__devtools, taken from the same base passed to createOnboarding. Connect strips the mount path, so the wrapper puts req.originalUrl back before calling the onboarding middleware.

The module test used a fake middlewares.use(fn) that skipped connect's routing, so it could not catch this. It now mounts the module on a real Vite server in middleware mode. It also checks that Nuxt's routing rule (route.length > 1 && url.startsWith(route)) sends /__devtools/embedded.js to Vite and that the script comes back as JavaScript. This adds vite as a devDependency of the package.

Verification

• pnpm vitest run packages/devtools-onboard: the new assertions pass. With the old mount restored, offers to install @nuxt/devtools when it is missing fails as expected.
• loads @nuxt/devtools when it is installed fails on main too, with or without this change. It likely needs @nuxt/devtools built first; not confirmed.
• pnpm eslint packages/devtools-onboard pnpm-workspace.yaml: clean.
• Not verified: the curl -i localhost:3000/__devtools/embedded.js repro against nuxt dev in the playground.

This PR was created with the help of an agent.

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