Add tools support to sampling (#976)

src/ModelContextProtocol.Core/Protocol/SingleItemOrListConverter.cs

@@ -0,0 +1,67 @@
+using System.ComponentModel;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// JSON converter for <see cref="IList{T}"/> that handles both array and single object representations.
+/// </summary>
+[EditorBrowsable(EditorBrowsableState.Never)]
+public sealed class SingleItemOrListConverter<T> : JsonConverter<IList<T>>
+    where T : class
+{
+    /// <inheritdoc />
+    public override IList<T>? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType == JsonTokenType.Null)
+        {
+            return null;
+        }
+
+        if (reader.TokenType == JsonTokenType.StartArray)
+        {
+            List<T> list = [];
+            while (reader.Read() && reader.TokenType != JsonTokenType.EndArray)
+            {
+                if (JsonSerializer.Deserialize(ref reader, options.GetTypeInfo(typeof(T))) is T item)
+                {
+                    list.Add(item);
+                }
+            }
+
+            return list;
+        }
+        
+        if (reader.TokenType == JsonTokenType.StartObject)
+        {
+            return JsonSerializer.Deserialize(ref reader, options.GetTypeInfo(typeof(T))) is T item ? [item] : [];
+        }
+
+        throw new JsonException($"Unexpected token type: {reader.TokenType}. Expected StartArray or StartObject.");
+    }
+
+    /// <inheritdoc />
+    public override void Write(Utf8JsonWriter writer, IList<T> value, JsonSerializerOptions options)
+    {
+        switch (value)
+        {
+            case null:
+                writer.WriteNullValue();
+                return;
+
+            case { Count: 1 }:
+                JsonSerializer.Serialize(writer, value[0], options.GetTypeInfo(typeof(object)));
+                return;
+
+            default:
+                writer.WriteStartArray();
+                foreach (var item in value)
+                {
+                    JsonSerializer.Serialize(writer, item, options.GetTypeInfo(typeof(object)));
+                }
+                writer.WriteEndArray();
+                return;
+        }
+    }
+}