Combining `values()` querysets on a model with `Meta.ordering` crashes

[Adam Johnson]

Combining querysets restricted with values()/values_list() (e.g. via union()) on a model with Meta.ordering crashes at SQL compilation on Django 6.1 when the default ordering columns are not part of the selected columns:

django.db.utils.DatabaseError: ORDER BY term does not match any column in the result set.

Minimal reproduction (crashes on PostgreSQL, MySQL, and Oracle — any backend with supports_slicing_ordering_in_compound=True; the same code works on Django 6.0):

class Book(models.Model):
    name = models.CharField(max_length=10)
    rank = models.IntegerField()

    class Meta:
        ordering = ["rank"]

Book.objects.values("name").union(Book.objects.values("name"))  # crashes on iteration

Same for values_list(), and for intersection()/difference(). qs1.union(qs2).values("name") doesn't crash but silently adds a spurious __orderbycol2 column to the compiled query and result set.

Regression in 087bb9e8f3478d53f12b1737af865992af17c5f2 (Refs #36644 -- Applied default ordering after union()), which sets default_ordering = True on the combined query so that Meta.ordering is applied after the combination. Root cause: for combined queries with a limited list of selected fields, SQLCompiler.get_order_by() cannot inject the implicit ordering columns into the values() parts (has_select_fields guard in django/db/models/sql/compiler.py), so it raises DatabaseError.