feat: implement upgrade validation to prevent downgrades and enhance service upgrade process with improved error handling

app/Models/Service.php

@@ -191,8 +191,14 @@ class Service extends Model
             // Only get the upgrades that have the exact same billing cycle as the service
             if ($plan->count() > 0) {
                 $product->plan = $plan->first();
-
-                return $product;
+                
+                // Only allow upgrades (not downgrades) - check if new price is greater than or equal to current price
+                $currentPrice = $this->plan->price()->price;
+                $newPrice = $product->plan->price()->price;
+                
+                if ($newPrice >= $currentPrice) {
+                    return $product;
+                }
             }
 
             return null;