Add generic UpdateCommand and handler using MediatR

Introduced abstract UpdateCommand<TUpdateDto, TEntity> and UpdateCommandHandler<TCommand, TUpdateDto, TEntity> in EnvelopeGenerator.Application.Common.Commands. This provides a reusable, type-safe pattern for update operations using a generic repository and MediatR, requiring implementers to supply an update DTO and a query expression for entity selection.

EnvelopeGenerator.Application/Common/Commands/UpdateCommand.cs

@@ -0,0 +1,59 @@
+using DigitalData.Core.Abstraction.Application.Repository;
+using MediatR;
+using System.Linq.Expressions;
+
+namespace EnvelopeGenerator.Application.Common.Commands;
+
+/// <summary>
+/// 
+/// </summary>
+/// <typeparam name="TUpdateDto"></typeparam>
+/// <typeparam name="TEntity"></typeparam>
+public abstract record UpdateCommand<TUpdateDto, TEntity> : IRequest where TUpdateDto : class where TEntity : class
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public TUpdateDto Update { get; init; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <returns></returns>
+    public abstract Expression<Func<TEntity, bool>> BuildQueryExpression();
+}
+
+/// <summary>
+/// 
+/// </summary>
+/// <typeparam name="TCommand"></typeparam>
+/// <typeparam name="TUpdateDto"></typeparam>
+/// <typeparam name="TEntity"></typeparam>
+public class UpdateCommandHandler<TCommand, TUpdateDto, TEntity> : IRequestHandler<TCommand>
+    where TUpdateDto : class
+    where TEntity : class
+    where TCommand : UpdateCommand<TUpdateDto, TEntity>
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    protected readonly IRepository<TEntity> Repository;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    public UpdateCommandHandler(IRepository<TEntity> repository)
+    {
+        Repository = repository;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    public Task Handle(TCommand request, CancellationToken cancel)
+        => Repository.UpdateAsync(request.Update, request.BuildQueryExpression(), cancel);
+}