.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/IAwaitableWorkflowRun.cs

@@ -0,0 +1,34 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Agents.AI.DurableTask.Workflows;
+
+/// <summary>
+/// Represents a workflow run that can be awaited for completion.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This interface extends <see cref="IWorkflowRun"/> to provide methods for waiting
+/// until the workflow execution completes. Not all workflow runners support this capability.
+/// </para>
+/// <para>
+/// Use pattern matching to check if a workflow run supports awaiting:
+/// <code>
+/// IWorkflowRun run = await client.RunAsync(workflow, input);
+/// if (run is IAwaitableWorkflowRun awaitableRun)
+/// {
+///     string? result = await awaitableRun.WaitForCompletionAsync&lt;string&gt;();
+/// }
+/// </code>
+/// </para>
+/// </remarks>
+public interface IAwaitableWorkflowRun : IWorkflowRun
+{
+    /// <summary>
+    /// Waits for the workflow to complete and returns the result.
+    /// </summary>
+    /// <typeparam name="TResult">The expected result type.</typeparam>
+    /// <param name="cancellationToken">A cancellation token to observe.</param>
+    /// <returns>The result of the workflow execution.</returns>
+    /// <exception cref="InvalidOperationException">Thrown when the workflow failed or was terminated.</exception>
+    ValueTask<TResult?> WaitForCompletionAsync<TResult>(CancellationToken cancellationToken = default);
+}