fix: data races in Square mock and scheduler hostname tests

- square_dev.go: return a copy of CheckoutResult to the caller
  instead of sharing the map-stored pointer, preventing the goroutine
  from racing with the caller on Status field reads
- scheduler_test.go: remove t.Parallel from TestInitHostname_* tests
  which modify a shared package-level hostname variable

backend/internal/square/square_dev.go

@@ -115,7 +115,7 @@ func (m *MockClient) CreateCheckout(ctx context.Context, req CreateCheckoutReq)
 	}
 
 	m.mu.Lock()
-	m.checkouts[checkoutID] = result
+	m.checkouts[checkoutID] = &CheckoutResult{ID: checkoutID, Status: "PENDING"}
 	m.mu.Unlock()
 
 	if !m.HoldCheckouts {