Add validation pipeline with FluentValidation and MediatR

Introduced a validation pipeline using FluentValidation and
MediatR to enhance request validation. Added the following:

- Registered FluentValidation and MediatR dependencies in
  `ReC.Application.csproj`.
- Updated `DependencyInjection.cs` to register validators
  and MediatR behaviors, including `ValidationBehavior<,>`.
- Added `ValidationBehavior<TRequest, TResponse>` to handle
  request validation in the MediatR pipeline.
- Created `ReadOutResQueryValidator` to enforce validation
  rules for `ReadOutResQuery`.
- Refactored namespaces and imports for better organization.

These changes improve extensibility, maintainability, and
separation of concerns in the application.

src/ReC.Application/Common/Behaviors/ValidationBehavior.cs

@@ -0,0 +1,30 @@
+using FluentValidation;
+using MediatR;
+
+namespace ReC.Application.Common.Behaviors
+{
+    public class ValidationBehavior<TRequest, TResponse>(IEnumerable<IValidator<TRequest>> validators) : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
+    {
+        public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancel)
+        {
+            if (validators.Any())
+            {
+                var context = new ValidationContext<TRequest>(request);
+
+                var validationResults = await Task.WhenAll(
+                    validators.Select(v =>
+                        v.ValidateAsync(context, cancel)));
+
+                var failures = validationResults
+                    .SelectMany(r => r.Errors)
+                    .Where(f => f != null)
+                    .ToList();
+
+                if (failures.Count != 0)
+                    throw new ValidationException(failures);
+            }
+            return await next(cancel);
+        }
+    }
+}

src/ReC.Application/DependencyInjection.cs

@@ -1,8 +1,10 @@
-using Microsoft.Extensions.Configuration;
+using MediatR;
+using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using ReC.Application.Common.Behaviors;
 using ReC.Application.Common.Options;
 using System.Reflection;
+using FluentValidation;
 
 namespace ReC.Application;
 
@@ -17,6 +19,8 @@ public static class DependencyInjection
 
         configOpt.ApplyConfigurations(services);
 
+        services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());
+
         services.AddAutoMapper(cfg =>
         {
             cfg.AddMaps(Assembly.GetExecutingAssembly());
@@ -27,6 +31,7 @@ public static class DependencyInjection
         {
             cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly());
             cfg.AddOpenBehaviors([typeof(BodyQueryBehavior<,>), typeof(HeaderQueryBehavior<,>)]);
+            cfg.AddBehavior(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));
             cfg.LicenseKey = configOpt.LuckyPennySoftwareLicenseKey;
         });
 

src/ReC.Application/OutResults/Queries/ReadOutResQueryValidator.cs

@@ -0,0 +1,13 @@
+using FluentValidation;
+
+namespace ReC.Application.OutResults.Queries;
+
+public class ReadOutResQueryValidator : AbstractValidator<ReadOutResQuery>
+{
+    public ReadOutResQueryValidator()
+    {
+        RuleFor(x => x)
+            .Must(x => x.ActionId.HasValue || x.ProfileId.HasValue)
+            .WithMessage("At least one of ActionId or ProfileId must be provided.");
+    }
+}

src/ReC.Application/ReC.Application.csproj

@@ -11,6 +11,8 @@
     <PackageReference Include="DigitalData.Core.Abstraction.Application" Version="1.5.0" />
     <PackageReference Include="DigitalData.Core.Application" Version="3.4.0" />
     <PackageReference Include="DigitalData.Core.Exceptions" Version="1.1.0" />
+    <PackageReference Include="FluentValidation" Version="12.1.0" />
+    <PackageReference Include="FluentValidation.DependencyInjectionExtensions" Version="12.1.0" />
     <PackageReference Include="MediatR" Version="13.1.0" />
     <PackageReference Include="Microsoft.EntityFrameworkCore" Version="9.0.11" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="9.0.11" />