Grant items to a player inventory using PlayFab Economy v2 AddInventoryItems

Steps

1. Obtain an Entity Session Token for the player via PlayFab authentication (LoginWithCustomID or equivalent)
2. POST to https://[titleId].playfabapi.com/Inventory/AddInventoryItems with the player EntityKey, a list of items (each with ItemId and Amount), and an optional StackId for targeting specific stacks
3. Handle the idempotent NewStackValues parameter when the item amount should initialise a new stack rather than merge into an existing one
4. Check the response TransactionIds array to correlate grants with audit logs; a single inventory is capped at 10,000 stacks
5. Call this only from a trusted server context; never expose AddInventoryItems to unauthenticated clients