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/DependencyInjection.cs

@@ -10,6 +10,8 @@ using System.Reflection;
 using MediatR;
 using EnvelopeGenerator.Application.DocReceiverElements.Commands;
 using EnvelopeGenerator.Application.DocReceiverElements.Behaviors;
+using EnvelopeGenerator.Application.Common.Behaviors;
+using FluentValidation;
 
 namespace EnvelopeGenerator.Application;
 
@@ -59,6 +61,9 @@ public static class DependencyInjection
         services.AddMediatR(cfg =>
         {
             cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly());
+
+            // Generic request validation
+            cfg.AddOpenBehavior(typeof(ValidationBehavior<,>));
             
             // Register SignCommand pipeline behaviors in execution order
             // 0. EnvelopeReceiverResolutionBehavior - Resolves EnvelopeReceiver from query parameters (executes FIRST)
@@ -80,6 +85,8 @@ public static class DependencyInjection
             cfg.AddBehavior<IPipelineBehavior<SigningCommand, Unit>, SendSignedMailBehavior>();
         });
 
+        services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());
+
         return services;
     }
 }