handle polymorphic_discriminator in query_expression

handle polymorphic_discriminator in query_expression

Added support for using :func:`_orm.with_expression` to populate a
:func:`_orm.query_expression` attribute that is also configured as the
``polymorphic_on`` discriminator column. The ORM now detects when a query
expression column is serving as the polymorphic discriminator and updates
it to use the column provided via :func:`_orm.with_expression`, enabling
polymorphic loading to work correctly in this scenario. This allows for
patterns such as where the discriminator value is computed from a related
table.

Fixes: #12631
Change-Id: I20baf4cddc5a19664bf73764f9371b187686af68