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

[idg10]

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.