middleware.AllowContentType not running for subroutes

Given this sample code

```go
package main

import (
"net/http"

"github.com/go-chi/chi/v5"
"github.com/go-chi/chi/v5/middleware"
)

func main() {
http.ListenAndServe(":3000", GetRouter())
}

func GetRouter() *chi.Mux {
apiRouter := chi.NewRouter()

apiRouter.Route("/foo-group", func(fooGroupRouter chi.Router) {
fooGroupRouter.Use(middleware.AllowContentType("application/json"))

fooGroupRouter.Post("/sub-route", HandleRoute( /* Params */))
})

// other routes

return apiRouter
}

func HandleRoute( /* Params */) http.HandlerFunc {
return func(responseWriter http.ResponseWriter, request *http.Request) {
responseWriter.WriteHeader(http.StatusCreated)

responseWriter.Write([]byte("done"))
}
}
```

When calling the API via

> POST localhost:3000/foo-group/sub-route

I get a 201 with "done". But I want to ensure this endpoint only accepts the content type "application/json", otherwise send back a 415.

Unfortunately the middleware is not working yet. I also tried to test the behaviour with the testrunner

```go
package main

import (
"net/http"
"net/http/httptest"
"strconv"
"testing"
)

func TestHandleRoute(suite *testing.T) {
server := httptest.NewServer(HandleRoute())

suite.Run("responds with status code "+strconv.Itoa(http.StatusUnsupportedMediaType)+" if content type is not application/json", func(testing *testing.T) {
response, _ := http.Post(server.URL, "text/xml", nil)

if response.StatusCode != http.StatusUnsupportedMediaType {
testing.Errorf("Expected statuscode %d but got %d", http.StatusUnsupportedMediaType, response.StatusCode)
}
})
}
```

Unfortunately the test fails with the message

> main_test.go:17: Expected statuscode 415 but got 201

so it seems the middleware didn't run.

Is my setup wrong or is it a bug?

Start by reproducing the request through GetRouter rather than testing HandleRoute directly, then inspect how middleware.AllowContentType is applied to the /foo-group route. Compare responses for application/json and text/xml, and confirm that the non-JSON request returns 415 while the accepted request still reaches HandleRoute.

Written by the indexing model from the issue text.