Add serialization roundtrip tests for all Protocol namespace types (#1289)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/ModelContextProtocol.Tests/Protocol/ElicitResultTests.cs

@@ -0,0 +1,51 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ElicitResultTests
+{
+    [Fact]
+    public static void ElicitResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new ElicitResult
+        {
+            Action = "accept",
+            Content = new Dictionary<string, JsonElement>
+            {
+                ["name"] = JsonDocument.Parse("\"John\"").RootElement.Clone(),
+                ["age"] = JsonDocument.Parse("30").RootElement.Clone()
+            },
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ElicitResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("accept", deserialized.Action);
+        Assert.True(deserialized.IsAccepted);
+        Assert.NotNull(deserialized.Content);
+        Assert.Equal(2, deserialized.Content.Count);
+        Assert.Equal("John", deserialized.Content["name"].GetString());
+        Assert.Equal(30, deserialized.Content["age"].GetInt32());
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void ElicitResult_SerializationRoundTrip_WithDefaultAction()
+    {
+        var original = new ElicitResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ElicitResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("cancel", deserialized.Action);
+        Assert.False(deserialized.IsAccepted);
+        Assert.Null(deserialized.Content);
+        Assert.Null(deserialized.Meta);
+    }
+}