fix(nuxt): Keep Nitro 2 WebSocket handlers resolvable with the managed tunnel - #26082

Why:
The tunnel plugin for Nitro 2 added in #25418 inserts a layer with an empty route '' at the top of the h3 application stack. The h3 WebSocket handler resolution logic skips layers with a route of '/', but it does not skip layers with ''. As a result, this layer matched all paths, preventing the app-defined WebSocket handler's upgrade hook from being called.

Changing the route to '/' causes this layer to be skipped during WebSocket handler resolution. For HTTP request processing, layers with a route of '/' continue to match all paths as well, so the tunnel behavior remains unchanged. In #25991, the maintainer approved this change as a temporary fix until #26001 is merged.

How to review:
The change in tunnel-legacy.server.ts is just a single line for the route value. The added test uses h3's createApp and createRouter to apply the plugin to an app where the router is mounted at /, just like Nitro. It then verifies the following three points:

• The return value of the WebSocket handler resolution process includes the app's upgrade hook.
• Requests to the tunnel are handled by the tunnel logic.
• Requests other than to the tunnel are handled by subsequent layers.

I have confirmed that the first test fails with the code before the fix.

Unsure about:
I have not added an E2E test using WebSockets in an actual Nuxt app. All I ran for verification were the unit tests for the nuxt package and the linting for the modified files.

Closes #25991