Add obsolete FirstAsync extension to TaskExtensions

Added FirstAsync<T, TException> as an obsolete extension method for Task<IEnumerable<T>>. This method returns the first element or throws a custom exception if the result is null, using a provided factory delegate. Intended for legacy .NET projects.

EnvelopeGenerator.Application/Common/Extensions/TaskExtensions.cs

@@ -65,6 +65,19 @@ public static class TaskExtensions
     /// <returns></returns>
     [Obsolete("Implement Mediator behaviors in the Osolete .NET project.")]
     public static Task<T?> FirstOrDefaultAsync<T>(this Task<IEnumerable<T>> task) => task.Then(t => t.FirstOrDefault());
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <typeparam name="T"></typeparam>
+    /// <typeparam name="TException"></typeparam>
+    /// <param name="task"></param>
+    /// <param name="factory"></param>
+    /// <returns></returns>
+    [Obsolete("Implement Mediator behaviors in the Osolete .NET project.")]
+    public static Task<T?> FirstAsync<T, TException>(this Task<IEnumerable<T>> task, Func<TException> factory) 
+        where TException : Exception 
+        => task.Then(t => t.FirstOrDefault()) ?? throw factory();
 }
 
 /// <summary>