.NET: [Feature Branch] Add basic durable workflow support (#3648)

* Add basic durable workflow support.

* PR feedback fixes

* Add conditional edge sample.

* PR feedback fixes.

* Minor cleanup.

* Minor cleanup

* Minor formatting improvements.

* Improve comments/documentation on the execution flow.

dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/EdgeRouters/IDurableEdgeRouter.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.Extensions.Logging;
+
+namespace Microsoft.Agents.AI.DurableTask.Workflows.EdgeRouters;
+
+/// <summary>
+/// Defines the contract for routing messages through workflow edges in durable orchestrations.
+/// </summary>
+/// <remarks>
+/// Implementations include <see cref="DurableDirectEdgeRouter"/> for single-target routing
+/// and <see cref="DurableFanOutEdgeRouter"/> for multi-target fan-out patterns.
+/// </remarks>
+internal interface IDurableEdgeRouter
+{
+    /// <summary>
+    /// Routes a message from the source executor to its target(s).
+    /// </summary>
+    /// <param name="envelope">The message envelope containing the message and metadata.</param>
+    /// <param name="messageQueues">The message queues to enqueue messages into.</param>
+    /// <param name="logger">The logger for tracing.</param>
+    void RouteMessage(
+        DurableMessageEnvelope envelope,
+        Dictionary<string, Queue<DurableMessageEnvelope>> messageQueues,
+        ILogger logger);
+}