Fix/pricing update fixes (#5275)

* fix for merging line items and quantity prices

* add changeset

* fix draft order unit_price calculation

* update test to reflect only quantity change

* fix unit tests

* update conditional

.changeset/many-peas-draw.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+fix(medusa): update unit_price for line-items with quantity pricing when merging on create