Fixed chats not loading in the v26.10.1 pre-release and added a trim smoke test (#1128)

app/TrimSmokeTest/JsonContractWalker.cs

@@ -0,0 +1,61 @@
+using System.Text.Json;
+using System.Text.Json.Serialization.Metadata;
+
+namespace AIStudio.TrimSmokeTest;
+
+/// <summary>
+/// Builds the contract System.Text.Json uses for a root and for everything it holds.
+/// </summary>
+/// <remarks>
+/// A type whose contract needs code the trimmer removed fails right here, before any value is read or
+/// written, just like it fails in AI Studio. A chat which lacks the property in question fails all the same.
+/// </remarks>
+public static class JsonContractWalker
+{
+    /// <summary>
+    /// Returns a description of every contract which cannot be built, starting at the given root.
+    /// </summary>
+    /// <param name="root">The type to start with, and the options AI Studio uses for it.</param>
+    /// <returns>One line per failure, naming the path from the root to the type which failed.</returns>
+    public static IReadOnlyList<string> FindBrokenContracts(JsonRoot root)
+    {
+        //
+        // The same System.Text.Json does with the first call of Serialize or Deserialize:
+        //
+        root.Options.MakeReadOnly(populateMissingResolver: true);
+
+        var failures = new List<string>();
+        Visit(root.Type, root.Type.Name, root.Options, [], failures);
+        return failures;
+    }
+
+    private static void Visit(Type type, string path, JsonSerializerOptions options, HashSet<Type> visited, List<string> failures)
+    {
+        if (!visited.Add(type))
+            return;
+
+        JsonTypeInfo typeInfo;
+        try
+        {
+            typeInfo = options.GetTypeInfo(type);
+        }
+        catch (Exception exception) when (exception is NotSupportedException or InvalidOperationException)
+        {
+            failures.Add($"{path}: {exception.Message}");
+            return;
+        }
+
+        foreach (var property in typeInfo.Properties)
+            Visit(property.PropertyType, $"{path} → {property.Name}", options, visited, failures);
+
+        if (typeInfo.KeyType is { } keyType)
+            Visit(keyType, $"{path} → key", options, visited, failures);
+
+        if (typeInfo.ElementType is { } elementType)
+            Visit(elementType, $"{path} → element", options, visited, failures);
+
+        if (typeInfo.PolymorphismOptions is { } polymorphism)
+            foreach (var derivedType in polymorphism.DerivedTypes)
+                Visit(derivedType.DerivedType, $"{path} → {derivedType.DerivedType.Name}", options, visited, failures);
+    }
+}