增加Mediat Pip特性，并用实例来做一个demo

Modules/Store.Users/Behaviors/AccountConsumptionBehavior.cs

@@ -0,0 +1,44 @@
+using MAX.Framework.Commons;
+using MAX.Framework.Mediation;
+using MediatR;
+using Store.Users.Entities;
+
+namespace Store.Users.Behaviors;
+
+/// <summary>锁定积分账户并检查消费额度，不依赖商铺信息。</summary>
+public sealed class AccountConsumptionBehavior<TRequest, TResponse>(
+    UnitOfWorkExecutionContext executionContext)
+    : IPipelineBehavior<TRequest, TResponse>
+    where TRequest : class, IRequest<TResponse>, IAccountConsumptionRequest
+{
+    public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken,
+        RequestHandlerDelegate<TResponse> next)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        if (string.IsNullOrWhiteSpace(request.UserName))
+            throw new AppException("用户名不能为空");
+        var userName = request.UserName.Trim();
+        var user = await executionContext.Orm.Select<UserEntity>().Where(a => a.UserName == userName)
+            .ForUpdate().FirstAsync(cancellationToken)
+            ?? throw new AppException("不存在的用户");
+        var quote = request.CalculateConsumption(user);
+        if (quote.Amount <= 0 || quote.UnitPrice <= 0)
+            throw new AppException("消费金额必须大于0");
+        if (user.Points < quote.Amount)
+            throw new AppException("余额不足");
+        lock (request)
+        {
+            if (request.ConsumptionContext != null)
+                throw new InvalidOperationException("同一个消费命令不能同时执行");
+            request.ConsumptionContext = new AccountConsumptionContext(user, quote);
+        }
+        try
+        {
+            return await next();
+        }
+        finally
+        {
+            request.ConsumptionContext = null;
+        }
+    }
+}