Fix Walk for routes with handlers and subrouters - #1146

Fixes #830

Reproduction

Register a handler and a subrouter on the same Route() pattern:

r.Route("/foo", func(r chi.Router) {
    r.Route("/bar", func(r chi.Router) {
        r.Get("/{id}", handler)
    })
    r.Get("/bar", handler)
})

GET /foo/bar is routable, but chi.Walk only reported GET /foo/bar/{id} and omitted GET /foo/bar.

Root cause

walk returned immediately after descending into route.SubRoutes, so any real handlers stored on that same route entry were skipped. The route tree also uses stub endpoints for mount connectors; those synthetic handlers need to remain hidden from public route listings.

Fix

Continue walking handlers after subroutes, while keeping subrouter middleware propagation separate. routes() now filters synthetic stub handlers but preserves the internal * handler used to carry inline middleware into subrouters.

Compatibility

No API changes and no new dependencies. This only makes Walk/Routes report an already-routable handler that was previously omitted; synthetic mount connector routes remain hidden.

Validation

• Regression test go test . -run TestWalkRouteWithHandlerAndSubrouter -count=1 fails without the fix: expected Walk to include GET /foo/bar, got map[GET /foo/bar/{id}:true].
• go build ./... passed.
• go vet ./... passed.
• gofmt -l . produced no output.
• go test ./... passed (github.com/go-chi/chi/v5, github.com/go-chi/chi/v5/middleware).