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/InitializeRequestParamsTests.cs

@@ -0,0 +1,42 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class InitializeRequestParamsTests
+{
+    [Fact]
+    public static void InitializeRequestParams_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new InitializeRequestParams
+        {
+            ProtocolVersion = "2024-11-05",
+            Capabilities = new ClientCapabilities
+            {
+                Roots = new RootsCapability { ListChanged = true },
+                Sampling = new SamplingCapability()
+            },
+            ClientInfo = new Implementation
+            {
+                Name = "test-client",
+                Version = "1.0.0"
+            },
+            Meta = new JsonObject { ["progressToken"] = "tok-1" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<InitializeRequestParams>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("2024-11-05", deserialized.ProtocolVersion);
+        Assert.NotNull(deserialized.Capabilities);
+        Assert.NotNull(deserialized.Capabilities.Roots);
+        Assert.True(deserialized.Capabilities.Roots.ListChanged);
+        Assert.NotNull(deserialized.Capabilities.Sampling);
+        Assert.Equal("test-client", deserialized.ClientInfo.Name);
+        Assert.Equal("1.0.0", deserialized.ClientInfo.Version);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("tok-1", (string)deserialized.Meta["progressToken"]!);
+    }
+}