MediatR behavior not working

program.cs

/*

Needs these package in the csproj

  <ItemGroup>
    <PackageReference Include="CSharpFunctionalExtensions" Version="2.38.1" />
    <PackageReference Include="FluentValidation.DependencyInjectionExtensions" Version="11.5.2" />
    <PackageReference Include="MediatR" Version="12.0.1" />  
    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="7.0.5" />
    <PackageReference Include="Swashbuckle.AspNetCore" Version="6.4.0" />
  </ItemGroup>

*/

using CSharpFunctionalExtensions;
using FluentValidation;
using FluentValidation.Results;
using MediatR;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

// Register our validators.
builder.Services.AddValidatorsFromAssemblyContaining<GetWeatherQuery.Validator>();

// Register our Mediator Handlers and behaviors.
builder.Services.AddMediatR(configuration =>
    configuration
        .RegisterServicesFromAssemblyContaining(typeof(Program))
        //.AddOpenBehavior(typeof(ValidationBehavior <,>))
        .AddBehavior<
            IPipelineBehavior<GetWeatherQuery.Query, Result<GetWeatherQuery.Response, ValidationFailed>>,
            ValidationBehavior<GetWeatherQuery.Query, GetWeatherQuery.Response>>()
);


var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

var summaries = new[]
{
    "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
};

app.MapGet("/weatherforecast", () =>
{
    var forecast = Enumerable.Range(1, 5).Select(index =>
        new WeatherForecast
        (
            DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
            Random.Shared.Next(-20, 55),
            summaries[Random.Shared.Next(summaries.Length)]
        ))
        .ToArray();
    return forecast;
})
.WithName("GetWeatherForecast")
.WithOpenApi();

app.MapGet("/weatherforecast/{city:required}", async (
    string city, 
    IMediator mediator, 
    CancellationToken cancellationToken) =>
    {
        var query = new GetWeatherQuery.Query(city);
        var result = await mediator.Send(query, cancellationToken);

        return TypedResults.Ok(result);
    })
.WithOpenApi();


app.Run();

internal record WeatherForecast(DateOnly Date, int TemperatureC, string? Summary)
{
    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
}

public class GetWeatherQuery
{
    public record Query(string City) : IRequest<Response>;

    public record Response(string City, int Temp, DateTime SomeDate);

    public class Handler : IRequestHandler<Query, Response>
    {
        public Task<Response> Handle(Query request, CancellationToken cancellationToken)
        {
            var response = new Response("New York", 20, DateTime.Now);
            return Task.FromResult(response);
        }
    }

    public class Validator : AbstractValidator<Query>
    {
        public Validator()
        {
            RuleFor(x => x.City)
                .NotEmpty()
                .MinimumLength(5)
                .WithMessage("City must be more than 5 characters.");
        }
    }
}

public record ValidationFailed
{
    public IDictionary<string, string[]> Errors { get; }

    public ValidationFailed(ValidationResult validationResult)
    {
        Errors = validationResult.ToDictionary();
    }
}

public class ValidationBehavior<TRequest, TResult> :
        IPipelineBehavior<TRequest, Result<TResult, ValidationFailed>> where TRequest : notnull
{
    private readonly IValidator<TRequest> _validator;

    public ValidationBehavior(IValidator<TRequest> validator)
    {
        _validator = validator;
    }

    public async Task<Result<TResult, ValidationFailed>> Handle(
        TRequest request,
        RequestHandlerDelegate<Result<TResult, ValidationFailed>> next,
        CancellationToken cancellationToken)
    {
        var validationResult = await _validator.ValidateAsync(request, cancellationToken);

        if (!validationResult.IsValid)
        {
            return new ValidationFailed(validationResult);
        }

        return await next();
    }
}