feat: add TaskExtensions with null and empty result checks

- Introduced ThrowIfNull<T>(Task<T?>) extension method to ensure awaited result is not null, throwing NotFoundException otherwise
- Added ThrowIfNull<T>(Task<IEnumerable<T>?>) extension method to validate collections, throwing NotFoundException if null or empty
- Supports optional custom exception messages for better error context

EnvelopeGenerator.Application/Extensions/TaskExtensions.cs

@@ -0,0 +1,39 @@
+using EnvelopeGenerator.Application.Exceptions;
+
+namespace EnvelopeGenerator.Application.Extensions;
+
+/// <summary>
+/// Extension methods for tasks
+/// </summary>
+public static class TaskExtensions
+{
+    /// <summary>
+    /// Awaits the specified task and ensures that the result is not <c>null</c>.
+    /// If the result is <c>null</c>, a <see cref="NotFoundException"/> is thrown.
+    /// </summary>
+    /// <typeparam name="T">The type of the result.</typeparam>
+    /// <param name="task">The task to await.</param>
+    /// <param name="exceptionMessage">Optional custom exception message.</param>
+    /// <returns>The awaited result if not <c>null</c>.</returns>
+    /// <exception cref="NotFoundException">Thrown if the result is <c>null</c>.</exception>
+    public static async Task<T> ThrowIfNull<T>(this Task<T?> task, string? exceptionMessage = null)
+    {
+        var result = await task;
+        return result ?? throw new NotFoundException(exceptionMessage);
+    }
+
+    /// <summary>
+    /// Awaits the specified task and ensures that the result is not <c>null</c> or empty.
+    /// If the result is <c>null</c> or contains no elements, a <see cref="NotFoundException"/> is thrown.
+    /// </summary>
+    /// <typeparam name="T">The element type of the collection.</typeparam>
+    /// <param name="task">The task to await.</param>
+    /// <param name="exceptionMessage">Optional custom exception message.</param>
+    /// <returns>The awaited collection if it is not <c>null</c> or empty.</returns>
+    /// <exception cref="NotFoundException">Thrown if the result is <c>null</c> or empty.</exception>
+    public static async Task<IEnumerable<T>> ThrowIfNull<T>(this Task<IEnumerable<T>?> task, string? exceptionMessage = null)
+    {
+        var result = await task;
+        return result?.Any() ?? false ? result : throw new NotFoundException(exceptionMessage);
+    }
+}