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/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.");
+    }
+}