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/CompleteResultTests.cs

@@ -0,0 +1,53 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class CompleteResultTests
+{
+    [Fact]
+    public static void CompleteResult_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new CompleteResult
+        {
+            Completion = new Completion
+            {
+                Values = ["weather", "web", "webhook"],
+                Total = 10,
+                HasMore = true
+            },
+            Meta = new JsonObject { ["key"] = "value" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CompleteResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Completion);
+        Assert.Equal(3, deserialized.Completion.Values.Count);
+        Assert.Equal("weather", deserialized.Completion.Values[0]);
+        Assert.Equal("web", deserialized.Completion.Values[1]);
+        Assert.Equal("webhook", deserialized.Completion.Values[2]);
+        Assert.Equal(10, deserialized.Completion.Total);
+        Assert.True(deserialized.Completion.HasMore);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("value", (string)deserialized.Meta["key"]!);
+    }
+
+    [Fact]
+    public static void CompleteResult_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new CompleteResult();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CompleteResult>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Completion);
+        Assert.Empty(deserialized.Completion.Values);
+        Assert.Null(deserialized.Completion.Total);
+        Assert.Null(deserialized.Completion.HasMore);
+        Assert.Null(deserialized.Meta);
+    }
+}