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/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;
         });