Add MediatR query/handler for reading envelope types

Introduced ReadEnvelopeTypesQuery and its handler to fetch all
envelope types from the repository, map them to DTOs, and
return the result via MediatR. This enables clean, decoupled
retrieval of envelope types in the application.

EnvelopeGenerator.Application/EnvelopeTypes/Queries/ReadEnvelopeTypesQuery.cs

@@ -0,0 +1,45 @@
+using EnvelopeGenerator.Application.Common.Dto;
+using MediatR;
+using AutoMapper;
+using DigitalData.Core.Abstraction.Application.Repository;
+using EnvelopeGenerator.Domain.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.EnvelopeTypes.Queries;
+
+/// <summary>
+/// 
+/// </summary>
+public record ReadEnvelopeTypesQuery : IRequest<IEnumerable<EnvelopeTypeDto>>;
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadEnvelopeTypesQueryHandler : IRequestHandler<ReadEnvelopeTypesQuery, IEnumerable<EnvelopeTypeDto>>
+{
+    private readonly IRepository<EnvelopeType> _repository;
+    private readonly IMapper _mapper;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    /// <param name="mapper"></param>
+    public ReadEnvelopeTypesQueryHandler(IRepository<EnvelopeType> repository, IMapper mapper)
+    {
+        _repository = repository;
+        _mapper = mapper;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<IEnumerable<EnvelopeTypeDto>> Handle(ReadEnvelopeTypesQuery request, CancellationToken cancellationToken)
+    {
+        var types = await _repository.Query.AsNoTracking().ToListAsync(cancellationToken);
+        return _mapper.Map<IEnumerable<EnvelopeTypeDto>>(types);
+    }
+}