Add request validation with FluentValidation

Introduced `ValidationBehavior` to validate MediatR requests using FluentValidation. Registered the behavior in the pipeline and added validators for `ReadDocumentDetailsQuery` and `ReadDocumentQuery`. Updated dependency injection to include validators and added FluentValidation NuGet packages. Improved exception handling with `BadRequestException` for validation failures. Refactored namespaces and imports to support the changes.

EnvelopeGenerator.Application/Common/Behaviors/ValidationBehavior.cs

@@ -0,0 +1,40 @@
+using DigitalData.Core.Exceptions;
+using FluentValidation;
+using MediatR;
+
+namespace EnvelopeGenerator.Application.Common.Behaviors;
+
+public class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+    where TRequest : notnull
+{
+    private readonly IEnumerable<IValidator<TRequest>> _validators;
+
+    public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
+    {
+        _validators = validators;
+    }
+
+    public async Task<TResponse> Handle(
+        TRequest request,
+        RequestHandlerDelegate<TResponse> next,
+        CancellationToken cancellationToken)
+    {
+        if (_validators.Any())
+        {
+            var context = new ValidationContext<TRequest>(request);
+            var failures = _validators
+                .Select(v => v.Validate(context))
+                .SelectMany(r => r.Errors)
+                .Where(f => f is not null)
+                .ToList();
+
+            if (failures.Count > 0)
+            {
+                var message = string.Join("; ", failures.Select(f => f.ErrorMessage).Distinct());
+                throw new BadRequestException(message);
+            }
+        }
+
+        return await next();
+    }
+}