Fixes for Turbopack HMR (#54790)
### What?
There were a few issues with the initial implementation of next-api HMR:
1. We incorrectly errored out when we received a Next.js WebSocket message
2. We didn't handle Next's `span-end` message, leading to another error
3. We listened to the `htmlEndpoint` change events instead of the `dataEndpoint`/`rscEndpoint`, leading to us detecting client-side changes and causing full page reloads
### Why?
HMR is the life-blood of development
### How?
Small fixes to our Turbopack reimplementation of the server-side HRM handlers
Closes WEB-1475
