fix(mediator): 修复参数验证错误消息中的字符串拼接问题

- 修正了 Name 参数为空时错误消息中的字符串拼接格式
- 在错误消息中添加了适当的空格分隔符

GFramework.Core.Tests/mediator/MediatorComprehensiveTests.cs

@@ -531,7 +531,7 @@ public sealed class TestValidatedCommandHandler : IRequestHandler<TestValidatedC
     {
         if (string.IsNullOrWhiteSpace(request.Name))
         {
-            throw new ArgumentException($"Name cannot be empty{nameof(request.Name)}");
+            throw new ArgumentException($"Name cannot be empty {nameof(request.Name)}");
         }
 
         return ValueTask.FromResult(Unit.Value);