feat(fifa17): add purchase_items to host CoreEconomy client

Complete the transport contract: purchase_items (atomic debit + mint N) on the
CoreEconomy trait + HttpCoreClient (POST /economy/purchase-items) + FakeEconomy.
This is the open-on-buy primitive the Store BUY handler will use (createPackResponse
returns the minted itemList). Fail-closed like the rest of the client.

openfut-utas-host/src/lib.rs

@@ -399,6 +399,8 @@ pub trait CoreEconomy: Send + Sync {
     fn sell_item(&self, item_id: &str, price: i64) -> Result<i64, CoreError>;
     fn grant_reward(&self, amount: i64) -> Result<i64, CoreError>;
     fn purchase_item(&self, cost: i64, item_id: &str, card_id: &str) -> Result<i64, CoreError>;
+    /// Debit `cost` and mint several items atomically (open-on-buy Store packs).
+    fn purchase_items(&self, cost: i64, items: &[EconomyGrantItem]) -> Result<i64, CoreError>;
 }
 
 impl HttpCoreClient {
@@ -517,6 +519,18 @@ impl CoreEconomy for HttpCoreClient {
         )?;
         json_i64(&v, "balance")
     }
+
+    fn purchase_items(&self, cost: i64, items: &[EconomyGrantItem]) -> Result<i64, CoreError> {
+        let items_json: Vec<Value> = items
+            .iter()
+            .map(|i| json!({ "item_id": i.item_id, "card_id": i.card_id }))
+            .collect();
+        let v = self.economy_post(
+            "purchase-items",
+            &json!({ "cost": cost, "items": items_json }),
+        )?;
+        json_i64(&v, "balance")
+    }
 }
 
 /// Serialize a [`CoreReplaceRequest`] into the `PUT /squad/replace` JSON body.
@@ -2215,6 +2229,16 @@ mod tests {
             }
             Ok(self.balance)
         }
+        fn purchase_items(
+            &self,
+            _cost: i64,
+            _items: &[EconomyGrantItem],
+        ) -> Result<i64, CoreError> {
+            if self.fail {
+                return Err(CoreError::Status(500));
+            }
+            Ok(self.balance)
+        }
     }
 
     #[test]