WebApplicationBuilder and IStartupFilter break routing if split between them

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

As described in my comment in: https://github.com/dotnet/aspnetcore/issues/44599

When calling UseRouting() in IStartupFilter, it will not see endpoints mapped by WebApplication.MapControllers().

```csharp
// Presume this is nuget
class FooStartupFilter: IStartupFilter {
public Action Configure(Action next)
=> builder => {
builder.UseRouting();
builder.Use(next => ctx => {
// do something with Endpoint.
});

next(builder);
}
}
```
```
var builder = ....; // get WebApplicationBuilder

builder.Services.AddTransient();

var webApplication = builder.Build();

webApplication .MapControllers();
```

Second middleware in IStartupFilter will not see any endpoints unless UseEndpoints(e => e.MapControllers()) is added.

### Expected Behavior

UseRouting in IStartupFilter should be able to access mapped routes that were mapped on WebAppliaction and vice versa.

### Steps To Reproduce

_No response_

### Exceptions (if any)

_No response_

### .NET Version

_No response_

### Anything else?

Most likely caused by WebApplicationBuilder.cs:395.