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/ReadResourceResultTests.cs

@@ -0,0 +1,64 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ReadResourceResultTests
+{
+    [Fact]
+    public static void ReadResourceResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new ReadResourceResult
+        {
+            Contents =
+            [
+                new TextResourceContents
+                {
+                    Uri = "file:///readme.md",
+                    MimeType = "text/markdown",
+                    Text = "# Hello"
+                },
+                new BlobResourceContents
+                {
+                    Uri = "file:///image.png",
+                    MimeType = "image/png",
+                    Blob = "base64data"
+                }
+            ],
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ReadResourceResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal(2, deserialized.Contents.Count);
+
+        var textContent = Assert.IsType<TextResourceContents>(deserialized.Contents[0]);
+        Assert.Equal("file:///readme.md", textContent.Uri);
+        Assert.Equal("text/markdown", textContent.MimeType);
+        Assert.Equal("# Hello", textContent.Text);
+
+        var blobContent = Assert.IsType<BlobResourceContents>(deserialized.Contents[1]);
+        Assert.Equal("file:///image.png", blobContent.Uri);
+        Assert.Equal("image/png", blobContent.MimeType);
+        Assert.Equal("base64data", blobContent.Blob);
+
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void ReadResourceResult_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new ReadResourceResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ReadResourceResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Empty(deserialized.Contents);
+        Assert.Null(deserialized.Meta);
+    }
+}