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/CompleteContextTests.cs

@@ -0,0 +1,41 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class CompleteContextTests
+{
+    [Fact]
+    public static void CompleteContext_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new CompleteContext
+        {
+            Arguments = new Dictionary<string, string>
+            {
+                ["language"] = "en",
+                ["region"] = "us"
+            }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CompleteContext>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Arguments);
+        Assert.Equal(2, deserialized.Arguments.Count);
+        Assert.Equal("en", deserialized.Arguments["language"]);
+        Assert.Equal("us", deserialized.Arguments["region"]);
+    }
+
+    [Fact]
+    public static void CompleteContext_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new CompleteContext();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CompleteContext>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Null(deserialized.Arguments);
+    }
+}