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/Documents/Queries/ReadDocumentDetailsQueryValidator.cs

@@ -0,0 +1,13 @@
+using FluentValidation;
+
+namespace EnvelopeGenerator.Application.Documents.Queries;
+
+public class ReadDocumentDetailsQueryValidator : AbstractValidator<ReadDocumentDetailsQuery>
+{
+    public ReadDocumentDetailsQueryValidator()
+    {
+        RuleFor(x => x.EnvelopeId)
+            .GreaterThan(0)
+            .WithMessage("EnvelopeId is required.");
+    }
+}