要在MediatR管道中调用FluentValidation，你可以使用以下解决方法：

1. 创建一个自定义的PipelineBehavior：

public class ValidationPipelineBehavior : IPipelineBehavior
{
    private readonly IEnumerable> _validators;

    public ValidationPipelineBehavior(IEnumerable> validators)
    {
        _validators = validators;
    }

    public async Task Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate next)
    {
        // 在调用处理程序之前执行验证
        var context = new ValidationContext(request);
        var validationResults = await Task.WhenAll(_validators.Select(v => v.ValidateAsync(context, cancellationToken)));
        var failures = validationResults.SelectMany(r => r.Errors).Where(f => f != null).ToList();

        if (failures.Count != 0)
        {
            throw new ValidationException(failures);
        }

        // 继续执行管道
        return await next();
    }
}

2. 在Startup.cs中注册自定义的PipelineBehavior：

public void ConfigureServices(IServiceCollection services)
{
    // 注册FluentValidation验证器
    services.AddValidatorsFromAssemblyContaining();

    // 注册自定义的PipelineBehavior
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationPipelineBehavior<,>));

    // 注册MediatR中的其他服务
    services.AddMediatR(typeof(Startup));
}

3. 在处理程序中添加验证器：

public class MyRequestValidator : AbstractValidator
{
    public MyRequestValidator()
    {
        RuleFor(x => x.Property1).NotNull().NotEmpty();
        RuleFor(x => x.Property2).GreaterThan(0);
    }
}

public class MyRequestHandler : IRequestHandler
{
    private readonly IMediator _mediator;

    public MyRequestHandler(IMediator mediator)
    {
        _mediator = mediator;
    }

    public async Task Handle(MyRequest request, CancellationToken cancellationToken)
    {
        // 调用处理程序之前会自动执行验证

        // 执行其他业务逻辑

        return new MyResponse();
    }
}

4. 在控制器或其他地方使用MediatR来处理请求：

public class MyController : ControllerBase
{
    private readonly IMediator _mediator;

    public MyController(IMediator mediator)
    {
        _mediator = mediator;
    }

    [HttpPost]
    public async Task MyAction([FromBody] MyRequest request)
    {
        var response = await _mediator.Send(request);
        return Ok(response);
    }
}

通过以上步骤，你就可以在MediatR管道中调用FluentValidation进行请求验证了。如果验证失败，将抛出ValidationException，并返回验证错误信息。