feat: expose `websocketPort` config option for reverse proxy compatibility

Clear and concise description of the problem

When the Vite dev server sits behind a reverse proxy (Caddy, nginx, etc.), the DevTools WebSocket connection fails. The proxy forwards HTTP traffic to Vite's main server port, but DevTools opens its own WebSocket server on a random port that the proxy doesn't know about.

createWsServer in packages/core/src/node/ws.ts picks the port with:

https://github.com/vitejs/devtools/blob/623fc58973bf44394572cecae27459c7c23d5401/packages/core/src/node/ws.ts#L31

The client reads /.devtools/.connection.json, gets {"backend":"websocket","websocket":62480} (or whatever random port was chosen), and tries to connect directly to that port. Through the proxy, this fails silently.

In my local dev environment, I use caddy to help simulate HTTPS and to have a more "production like" networking experience while still having HMR and other conveniences.

Suggested solution

Add a websocketPort option to DevToolsConfig in packages/core/src/node/config.ts and wire it through to createDevToolsMiddleware.

The internal plumbing already supports it — CreateWsServerOptions already accepts portWebSocket. It's just not exposed through the config or passed by DevToolsServer in packages/core/src/node/plugins/server.ts.

The change would be:

1. Add websocketPort?: number to DevToolsConfig
2. Include it in normalizeDevToolsConfig
3. In DevToolsServer.configureServer, read it from the resolved config and pass it as portWebSocket to createDevToolsMiddleware

Usage:

// vite.config.ts
export default defineConfig({
  devtools: {
    enabled: true,
    websocketPort: 7812,
  },
})

With a fixed port, users can configure their proxy to forward that port alongside Vite's main port.

Alternative

DevTools could reuse Vite's existing HMR WebSocket connection instead of opening a separate server. This would make it work behind any proxy that already supports Vite's dev server with no extra config. That's a much larger architectural change though — a config option for the port is a simpler path.

Additional context

In the meantime I have made for myself a small Vite plugin that intercepts requests to the DevTools inject script and returns an empty module when the Host header isn't localhost. This means DevTools only works on localhost — not through the proxy. Otherwise I will end up seeing Vite DevTools but would never be able to authenticate because it could never trigger an auth flow for me to see on the terminal.

As a side note:
The "Code of Conduct" link goes to a 404 but I checked it anyways.

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start with packages/core/src/node/config.ts and packages/core/src/node/plugins/server.ts, then inspect createDevToolsMiddleware and the existing portWebSocket option in packages/core/src/node/ws.ts. Add the websocketPort configuration flow described in the issue, and verify that a fixed port reaches the WebSocket server for proxy configuration.