Routes added via WebApplication get attached to the last UseRouting call

[davidfowl]

Describe the bug

Today we add routes from the WebApplication we copy the route data sources to the last call to UseEndpoints, this results in bizzare behavior when adding multiple calls to UseRouting and UseEndpoints.

To Reproduce

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.UseRouting();

app.Use((context, next) =>
{
    Console.WriteLine("Selected endpoint is " + (context.GetEndpoint()?.DisplayName is string s ? s : "null"));
    return next(context);
});

app.MapGet("/", () => "Hello World");

app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("/hi", () => "Hello Endpoints");
});

app.UseRouting();
app.UseEndpoints(_ => { });

app.Run();

If you request / you'll see "Selected endpoints is null". It's because the middleware runs after the first UseRouting middleware but the route is matched in the second UseRouting middleware.

[davidfowl]

We plan to treat the WebApplication as the global router for the entire pipeline so UseRouting will always use the same set of endpoints and will not create a new DefaultEndpointRouteBuilder per call.