fix(orders): Initialize WooCommerce cart/session before use

## Issue:
500 error on shipping/calculate and orders/preview endpoints
Error: "Call to a member function empty_cart() on null"

## Root Cause:
WC()->cart is not initialized in admin/REST API context
Calling WC()->cart->empty_cart() fails when cart is null

## Solution:
Initialize WooCommerce cart and session before using:

```php
// Initialize if not already loaded
if ( ! WC()->cart ) {
    wc_load_cart();
}
if ( ! WC()->session ) {
    WC()->session = new \WC_Session_Handler();
    WC()->session->init();
}

// Now safe to use
WC()->cart->empty_cart();
```

## Changes:
- Added initialization in calculate_shipping()
- Added initialization in preview_order()
- Both methods now safely use WC()->cart

## Testing:
- ✅ Endpoints no longer return 500 error
- ✅ Cart operations work correctly
- ✅ Session handling works in admin context

includes/Api/OrdersController.php

@@ -1236,6 +1236,15 @@ class OrdersController {
     }
 
     try {
+      // Initialize WooCommerce cart and session if not already
+      if ( ! WC()->cart ) {
+        wc_load_cart();
+      }
+      if ( ! WC()->session ) {
+        WC()->session = new \WC_Session_Handler();
+        WC()->session->init();
+      }
+      
       // Create a temporary cart to calculate shipping
       WC()->cart->empty_cart();
       
@@ -1319,6 +1328,15 @@ class OrdersController {
     }
 
     try {
+      // Initialize WooCommerce cart and session if not already
+      if ( ! WC()->cart ) {
+        wc_load_cart();
+      }
+      if ( ! WC()->session ) {
+        WC()->session = new \WC_Session_Handler();
+        WC()->session->init();
+      }
+      
       // Use WooCommerce cart for calculation
       WC()->cart->empty_cart();