`SetOp` and `EmptyTable` should not implement `TableIdNode`

A `SetOp` node's exec indexes are assigned based on its left child node rather than the columns set in `ColSet` (see dolthub/go-mysql-server#3110)

The `TableIdNode` functions implemented for `EmptyTable` seem to only be used for testing purposes and `cols` is never actually set. As a result, no ColumnId's are added during `columnIdsForNode`, causing an offset when assigning column indexes (see #10434)

The issue points to SetOp, EmptyTable, TableIdNode, and columnIdsForNode; begin by tracing how those nodes contribute column IDs and how SetOp assigns exec indexes. Confirm the behavior described in #10434 and dolthub/go-mysql-server#3110, then add focused regression coverage showing correct offsets and no unintended TableIdNode behavior.

Written by the indexing model from the issue text.