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);
+        }
+    }
+}