Use AuthorizationException for domain-specific errors

Replaced UnauthorizedAccessException with a custom
AuthorizationException in SignPdfByEnvelopeCommandHandler
to provide more meaningful error handling. Added the
EnvelopeGenerator.Domain.Exceptions namespace to support
this change.

EnvelopeGenerator.Application/PdfSigning/SignPdfByEnvelopeCommand.cs

@@ -2,6 +2,7 @@
 using DigitalData.Core.Exceptions;
 using EnvelopeGenerator.Application.Common.Query;
 using EnvelopeGenerator.Domain.Entities;
+using EnvelopeGenerator.Domain.Exceptions;
 using MediatR;
 using Microsoft.EntityFrameworkCore;
 
@@ -55,7 +56,7 @@ public class SignPdfByEnvelopeCommandHandler(IRepository<Envelope> Repo, ISender
             ?? throw new NotFoundException($"Envelope is not found ID: {request.Id} UUID: {request.Uuid}");
 
         if (request.UserId is int userId && userId != envelope.UserId)
-            throw new UnauthorizedAccessException($"Envelope is not authorized for user with Id: {request.UserId} Envelope ID: {request.Id} UUID: {request.Uuid}");
+            throw new AuthorizationException($"Envelope is not authorized for user with Id: {request.UserId} Envelope ID: {request.Id} UUID: {request.Uuid}");
 
         if (envelope.EnvelopeReceivers.Count == 0)
             throw new NotFoundException($"Envelope has no receivers to sign ID: {request.Id} UUID: {request.Uuid}");