fix(medusa): Account for multiple inventory items in get-inventory (#3094)

* use pvi service method instead of inventory service method to get quantity for sales channel

* save a few invocations

* rename quantity

* omit || 0

* add changeset

* extract method for retrieving variant inventory quantity

* update pr with feedback

.changeset/violet-pans-greet.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+fix(medusa): Account for multiple inventory items when getting inventory