allow batch with upsert if embed_values_counter is True

allow batch with upsert if embed_values_counter is True

Fixed issue in the :ref:`engine_insertmanyvalues` feature where using
PostgreSQL's ``ON CONFLICT`` clause with
:paramref:`_dml.Insert.returning.sort_by_parameter_order` enabled would
generate invalid SQL when the insert used an implicit sentinel (server-side
autoincrement primary key). The generated SQL would incorrectly declare a
sentinel counter column in the ``imp_sen`` table alias without providing
corresponding values in the ``VALUES`` clause, leading to a
``ProgrammingError`` indicating column count mismatch. The fix allows batch
execution mode when ``embed_values_counter`` is active, as the embedded
counter provides the ordering capability needed even with upsert behaviors,
rather than unnecessarily downgrading to row-at-a-time execution.

Fixes: #13107
Change-Id: I382472b2cf2991b520344adea5783584e27425d0
(cherry picked from commit 574facaaf4207b952379c28673c44b62835535fb)