better ucp conformance

internal/ucp/checkout.go

@@ -45,12 +45,70 @@ func (s *CheckoutServer) handleCreateCheckout(w http.ResponseWriter, r *http.Req
 		writeError(w, http.StatusBadRequest, "invalid request body: "+err.Error())
 		return
 	}
-	if req.CartId == "" {
-		writeError(w, http.StatusBadRequest, "cartId is required")
+
+	items := req.Items
+	if len(items) == 0 && len(req.LineItems) > 0 {
+		items = req.LineItems
+	}
+
+	var cartIdStr string
+	if req.CartId != "" {
+		cartIdStr = req.CartId
+	} else if len(items) > 0 {
+		// Fetch cart base URL
+		cartBaseURL := os.Getenv("CART_INTERNAL_URL")
+		if cartBaseURL == "" {
+			cartBaseURL = "http://cart:8080"
+		}
+
+		// Prepare POST request to /ucp/v1/carts/ to create and seed the cart
+		createReqBody := CreateCartRequest{
+			Items: items,
+		}
+		bodyBytes, err := json.Marshal(createReqBody)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to marshal create cart request: "+err.Error())
+			return
+		}
+
+		client := &http.Client{Timeout: 10 * time.Second}
+		cartReq, err := http.NewRequestWithContext(r.Context(), "POST", fmt.Sprintf("%s/ucp/v1/carts/", cartBaseURL), bytes.NewReader(bodyBytes))
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to create post cart request: "+err.Error())
+			return
+		}
+		cartReq.Header.Set("Content-Type", "application/json")
+
+		profile := os.Getenv("UCP_AGENT_PROFILE")
+		if profile == "" {
+			profile = DefaultUCPAgentProfile
+		}
+		cartReq.Header.Set("UCP-Agent", `profile="`+profile+`"`)
+
+		cartResp, err := client.Do(cartReq)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to create implicit cart: "+err.Error())
+			return
+		}
+		defer cartResp.Body.Close()
+
+		if cartResp.StatusCode != http.StatusCreated && cartResp.StatusCode != http.StatusOK {
+			writeError(w, http.StatusBadRequest, fmt.Sprintf("cart service returned status: %d when creating implicit cart", cartResp.StatusCode))
+			return
+		}
+
+		var cartRespBody CartResponse
+		if err := json.NewDecoder(cartResp.Body).Decode(&cartRespBody); err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to decode implicit cart response: "+err.Error())
+			return
+		}
+		cartIdStr = cartRespBody.Id
+	} else {
+		writeError(w, http.StatusBadRequest, "cartId or items/line_items is required")
 		return
 	}
 
-	cartId, ok := cart.ParseCartId(req.CartId)
+	cartId, ok := cart.ParseCartId(cartIdStr)
 	if !ok {
 		writeError(w, http.StatusBadRequest, "invalid cartId")
 		return