Initial draft of actor runtime abstractions (#197)

* Initial draft of actor runtime abstractions

dotnet/src/Microsoft.Extensions.AI.Agents.Runtime/JsonSerializerExtensions.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
+
+namespace Microsoft.Extensions.AI.Agents.Runtime;
+
+/// <summary>
+/// Provides extension methods for JSON serialization with source generation support.
+/// </summary>
+internal static class JsonSerializerExtensions
+{
+    /// <summary>
+    /// Gets the JsonTypeInfo for a type, preferring the one from options if available,
+    /// otherwise falling back to the source-generated context.
+    /// </summary>
+    /// <typeparam name="T">The type to get JsonTypeInfo for.</typeparam>
+    /// <param name="options">The JsonSerializerOptions to check first.</param>
+    /// <param name="fallbackContext">The fallback JsonSerializerContext to use if not found in options.</param>
+    /// <returns>The JsonTypeInfo for the requested type.</returns>
+    public static JsonTypeInfo<T> GetTypeInfo<T>(this JsonSerializerOptions options, JsonSerializerContext fallbackContext)
+    {
+        // Try to get from the options first (if a context is configured)
+        if (options.TypeInfoResolver?.GetTypeInfo(typeof(T), options) is JsonTypeInfo<T> typeInfo)
+        {
+            return typeInfo;
+        }
+
+        // Fall back to the provided source-generated context
+        return (JsonTypeInfo<T>)fallbackContext.GetTypeInfo(typeof(T))!;
+    }
+}