Queryable support post-System.Linq.Async isn't quite right

We have some issues:

* `System.Interactive.Async.Providers` has an implicit reference to `System.Linq.Async`
* `AsAsyncQueryable` requires `System.Linq.Async.Queryable, which brings an implicit reference to `System.Linq.Async`

It's not clear what the correct solution is. .NET 10.0 hasn't defined an equivalent for `System.Linq.Async.Queryable`. Should that library now become the queryable provider for `System.Linq.AsyncEnumerable`? Or does it become a legacy component to join `System.Linq.Async`, and we do something else?

The weird way we've had to use `AsyncEnumerableEx.SumAsync` in some places in tests is a side effect of this problem.