feat: Implement inventory policy for product variants

- Introduced InventoryPolicy enum to manage tracked and unlimited inventory types.
- Updated ProductVariant model to include inventory_policy and cantidad_vendida attributes.
- Modified addItem and updateItem methods in Cart to check available quantity based on inventory policy.
- Added migration to include inventory_policy and cantidad_vendida in productos_variantes table.
- Enhanced tests to validate inventory behavior for both tracked and unlimited policies.
- Updated various request and resource classes to handle new inventory fields.

database/migrations/2026_07_15_000000_add_inventory_policy_and_cantidad_vendida_to_productos_variantes_table.php

@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('productos_variantes', function (Blueprint $table) {
+            $table->string('inventory_policy')->default('tracked')->after('producto_id');
+            $table->unsignedBigInteger('cantidad_vendida')->default(0)->after('stock_reservado');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('productos_variantes', function (Blueprint $table) {
+            $table->dropColumn(['inventory_policy', 'cantidad_vendida']);
+        });
+    }
+};