Fix handling of hot reloader middlewares (#71104)
For every dev overlay middleware request (i.e.
`/__nextjs_original-stack-frame` or `/__nextjs_launch-editor`) we are:
- creating a forever-hanging promise in the Webpack hot reloader,
- unnecessarily running through the `handleRequest` logic in the router
server with the Turbopack hot reloader.
This PR fixes these two isses by correctly setting the `finished`
status, when a hot reloader middleware has not called `next()`.
