feat(sorting): implement ApplyOrderWithInStockPriority method for enhanced product sorting

- Added ApplyOrderWithInStockPriority method to prioritize in-stock products in queries.
- Updated GetDiscountProductsQueryHandler and GetCustomerProductsByFilterQueryHandler to utilize the new sorting method, ensuring in-stock items are listed first followed by user-defined sorting or creation date.
- Improved overall query handling for product availability and sorting logic.

src/CMSMicroservice.Application/Common/Extensions/SortByExtensions.cs

@@ -1,3 +1,4 @@
+using System.Linq.Expressions;
 using System.Linq.Dynamic.Core;
 using CMSMicroservice.Domain.Common;
 
@@ -16,6 +17,21 @@ public static class SortByExtensions
         return source.OrderBy(NormalizeSortBy(sortBy));
     }
 
+    /// <summary>
+    /// موجودی‌دارها اول، سپس سورت درخواستی کاربر (یا Created desc اگر SortBy خالی باشد).
+    /// </summary>
+    public static IQueryable<TSource> ApplyOrderWithInStockPriority<TSource>(
+        this IQueryable<TSource> source,
+        string? sortBy,
+        Expression<Func<TSource, bool>> isInStock) where TSource : BaseAuditableEntity
+    {
+        var ordered = source.OrderByDescending(isInStock);
+        if (string.IsNullOrWhiteSpace(sortBy))
+            return ordered.ThenByDescending(p => p.Created);
+
+        return ordered.ThenBy(NormalizeSortBy(sortBy));
+    }
+
     /// <summary>
     /// Dynamic LINQ treats "-Created" as unary minus on DateTime (invalid).
     /// Project convention: leading '-' means descending sort.