chore: import upstream snapshot with attribution

dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableMessageEnvelope.cs

@@ -0,0 +1,51 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Agents.AI.DurableTask.Workflows;
+
+/// <summary>
+/// Represents a message envelope for durable workflow message passing.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This is the durable equivalent of <c>MessageEnvelope</c> in the in-process runner.
+/// Unlike the in-process version which holds native .NET objects, this envelope
+/// contains serialized JSON strings suitable for Durable Task activities.
+/// </para>
+/// </remarks>
+internal sealed class DurableMessageEnvelope
+{
+    /// <summary>
+    /// Gets or sets the serialized JSON message content.
+    /// </summary>
+    public required string Message { get; init; }
+
+    /// <summary>
+    /// Gets or sets the full type name of the message for deserialization.
+    /// </summary>
+    public string? InputTypeName { get; init; }
+
+    /// <summary>
+    /// Gets or sets the ID of the executor that produced this message.
+    /// </summary>
+    /// <remarks>
+    /// Used for tracing and debugging. Null for initial workflow input.
+    /// </remarks>
+    public string? SourceExecutorId { get; init; }
+
+    /// <summary>
+    /// Creates a new message envelope.
+    /// </summary>
+    /// <param name="message">The serialized JSON message content.</param>
+    /// <param name="inputTypeName">The full type name of the message for deserialization.</param>
+    /// <param name="sourceExecutorId">The ID of the executor that produced this message, or null for initial input.</param>
+    /// <returns>A new <see cref="DurableMessageEnvelope"/> instance.</returns>
+    internal static DurableMessageEnvelope Create(string message, string? inputTypeName, string? sourceExecutorId = null)
+    {
+        return new DurableMessageEnvelope
+        {
+            Message = message,
+            InputTypeName = inputTypeName,
+            SourceExecutorId = sourceExecutorId
+        };
+    }
+}