feat: enhance product detail retrieval to support variant selection and update resource fields

app/Domains/Catalog/Services/ProductService.php

@@ -279,7 +279,7 @@ class ProductService
         return $products;
     }
 
-    public function getProductDetail(Tenant $tenant, Product $product): Product
+    public function getProductDetail(Tenant $tenant, Product $product, ?int $variantId = null): Product
     {
         $product->load([
             'attachments',
@@ -288,15 +288,27 @@ class ProductService
             'attributes.options',
         ]);
 
-        $defaultVariant = $product->variants()
+        $selectedVariantQuery = $product->variants()
             ->with([
                 'attachments',
                 'definitions.attribute.options',
-            ])
-            ->first();
+            ]);
 
-        if ($defaultVariant) {
-            $product->setDefaultVariant($defaultVariant);
+        $selectedVariant = $variantId !== null
+            ? $selectedVariantQuery->whereKey($variantId)->first()
+            : null;
+
+        if ($selectedVariant === null) {
+            $selectedVariant = $product->variants()
+                ->with([
+                    'attachments',
+                    'definitions.attribute.options',
+                ])
+                ->first();
+        }
+
+        if ($selectedVariant) {
+            $product->setSelectedVariant($selectedVariant);
         }
 
         return $product;