新增库位下拉接口及相关DTO与错误提示优化

新增 GetLocationOption 接口，完善 Controller、Service 层及 MmLocationOption DTO，实现库位下拉数据获取。修正仓库或库位不存在时的错误提示信息。

DOAN.Model/MES/Material/Dto/MmLocationDto.cs

@@ -50,4 +50,16 @@ namespace DOAN.Model.BZFM.Dto
         [ExcelColumn(Name = "库位类型(半成品/成品/临时/返工/报废)")]
         public string LocationTypeLabel { get; set; }
     }
+
+    /// <summary>
+    /// 库位下拉选择
+    /// </summary>
+    public class MmLocationOption
+    {
+        public string LocationCode { get; set; }
+        public string LocationName { get; set; }
+        public string WarehouseCode { get; set; }
+        public string WarehouseName { get; set; }
+        public string LocationType { get; set; }
+    }
 }