opt: incorrect costing of some streaming set ops

There are two cases where the optimizer will plan streaming set operators:

1. When the required ordering of the set op can be provided by both inputs (I think).
2. When the inputs provide matching interesting orderings.

In the case of (2), the `SetPrivate.Ordering` field is populated with the streaming ordering. In (1) it is not. We use the `SetPrivate.Ordering` to determine if a set op is streaming or not:

https://github.com/cockroachdb/cockroach/blob/17d0e21c4dba69e2f483f35ddbca8d012c59773c/pkg/sql/opt/xform/coster.go#L1388-L1394

So in the case of (1) we are incorrectly costing it as a non-streaming set op.

I also find it very confusing that for (1) there is no indication in the formatted optimizer expression that the set op is streaming. And in (2), the only indication is the `internal-ordering` field. I think that should be made very obvious in both cases, if possible.

Jira issue: CRDB-50439