error-or: A simple, fluent discriminated union of an error or a result.

public class ValidationBehavior<TRequest, TResponse>(IValidator<TRequest>? validator = null)
    : IPipelineBehavior<TRequest, TResponse>
        where TRequest : IRequest<TResponse>
        where TResponse : IErrorOr
{
    private readonly IValidator<TRequest>? _validator = validator;

    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken)
    {
        if (_validator is null)
        {
            return await next();
        }

        var validationResult = await _validator.ValidateAsync(request, cancellationToken);

        if (validationResult.IsValid)
        {
            return await next();
        }

        var errors = validationResult.Errors
            .ConvertAll(error => Error.Validation(
                code: error.PropertyName,
                description: error.ErrorMessage));

        return (dynamic)errors;
    }
}