增加Mediat Pip特性，并用实例来做一个demo

Core/MAX.Framework/Mediation/TransactionBehavior.cs

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+using FreeSql;
+using MediatR;
+
+namespace MAX.Framework.Mediation;
+
+/// <summary>为通过特性选择的命令管理事务；所有读写使用执行上下文中的 ORM。</summary>
+public sealed class TransactionBehavior<TRequest, TResponse>(
+    IFreeSql freeSql, UnitOfWorkExecutionContext context) : IPipelineBehavior<TRequest, TResponse>
+    where TRequest : IRequest<TResponse>
+{
+    public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken,
+        RequestHandlerDelegate<TResponse> next)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        if (context.IsActive)
+            throw new InvalidOperationException("不支持嵌套事务命令，请在同一个命令处理器内完成事务操作");
+        using var work = freeSql.CreateUnitOfWork();
+        work.GetOrBeginTransaction();
+        using var execution = context.Enter(work.Orm);
+        var response = await next();
+        cancellationToken.ThrowIfCancellationRequested();
+        work.Commit();
+        return response;
+        // 未提交时，工作单元释放自动回滚；执行上下文同时清理。
+    }
+}

Core/MAX.Framework/Mediation/UnitOfWorkExecutionContext.cs

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+using System.Threading;
+
+namespace MAX.Framework.Mediation;
+
+/// <summary>沿当前异步调用链传递事务 ORM，隔离并行命令的执行状态。</summary>
+public sealed class UnitOfWorkExecutionContext
+{
+    private readonly AsyncLocal<IFreeSql?> _current = new();
+
+    public IFreeSql Orm => _current.Value
+        ?? throw new InvalidOperationException("当前操作必须在事务管道内执行");
+
+    public bool IsActive => _current.Value != null;
+
+    internal IDisposable Enter(IFreeSql orm)
+    {
+        if (IsActive)
+            throw new InvalidOperationException("不支持嵌套事务命令，请在同一个命令处理器内完成事务操作");
+        _current.Value = orm;
+        return new ExecutionScope(this);
+    }
+
+    private sealed class ExecutionScope(UnitOfWorkExecutionContext context) : IDisposable
+    {
+        public void Dispose() => context._current.Value = null;
+    }
+}

Core/MAX.Framework/Mediation/ValidationBehavior.cs

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+using MAX.Core.Mediation;
+using MediatR;
+
+namespace MAX.Framework.Mediation;
+
+/// <summary>执行请求自身的参数校验，验证通过后才进入后续管道。</summary>
+public sealed class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+    where TRequest : IRequest<TResponse>, IValidatableRequest
+{
+    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken,
+        RequestHandlerDelegate<TResponse> next)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+        cancellationToken.ThrowIfCancellationRequested();
+        request.Validate();
+        return next();
+    }
+}