.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/DurableWorkflowJsonContext.cs

@@ -0,0 +1,34 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Text.Json.Serialization;
+
+namespace Microsoft.Agents.AI.DurableTask.Workflows;
+
+/// <summary>
+/// Source-generated JSON serialization context for durable workflow types.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This context provides AOT-compatible and trimmer-safe JSON serialization for the
+/// internal data transfer types used by the durable workflow infrastructure:
+/// </para>
+/// <list type="bullet">
+/// <item><description><see cref="DurableActivityInput"/>: Activity input wrapper with state</description></item>
+/// <item><description><see cref="DurableActivityOutput"/>: Activity output wrapper with results and events</description></item>
+/// <item><description><see cref="SentMessageInfo"/>: Messages sent via SendMessageAsync</description></item>
+/// </list>
+/// <para>
+/// Note: User-defined executor input/output types still use reflection-based serialization
+/// since their types are not known at compile time.
+/// </para>
+/// </remarks>
+[JsonSourceGenerationOptions(
+    WriteIndented = false,
+    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
+[JsonSerializable(typeof(DurableActivityInput))]
+[JsonSerializable(typeof(DurableActivityOutput))]
+[JsonSerializable(typeof(SentMessageInfo))]
+[JsonSerializable(typeof(List<SentMessageInfo>))]
+[JsonSerializable(typeof(Dictionary<string, string?>))]
+internal partial class DurableWorkflowJsonContext : JsonSerializerContext;