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/ListPromptsResultTests.cs

@@ -0,0 +1,58 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ListPromptsResultTests
+{
+    [Fact]
+    public static void ListPromptsResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new ListPromptsResult
+        {
+            Prompts =
+            [
+                new Prompt
+                {
+                    Name = "code_review",
+                    Title = "Code Review",
+                    Description = "Reviews code changes"
+                },
+                new Prompt
+                {
+                    Name = "summarize",
+                    Description = "Summarizes text"
+                }
+            ],
+            NextCursor = "page-2-token",
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListPromptsResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal(2, deserialized.Prompts.Count);
+        Assert.Equal("code_review", deserialized.Prompts[0].Name);
+        Assert.Equal("Code Review", deserialized.Prompts[0].Title);
+        Assert.Equal("summarize", deserialized.Prompts[1].Name);
+        Assert.Equal("page-2-token", deserialized.NextCursor);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void ListPromptsResult_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new ListPromptsResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ListPromptsResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Empty(deserialized.Prompts);
+        Assert.Null(deserialized.NextCursor);
+        Assert.Null(deserialized.Meta);
+    }
+}