Add upsert regression test for non-join partition changes

[ndrluis]

Rationale for this change

While reviewing #3387, I found a missing regression test for upsert behavior when a table is partitioned by a column that is not part of join_cols, and the source row changes that partition column value.

For example, if the table is partitioned by order_date but the upsert matches rows using only order_id, an existing row should still be matched by order_id even when the incoming row has a different order_date. The expected behavior is that the row is updated and moved to the new partition, not treated as a new insert.

Regardless of whether #3387 is merged, I believe this behavior should be protected in the repository because partition pruning optimizations in this area can otherwise accidentally prune away the existing target row before the join-key match is evaluated.