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/CompletionTests.cs

@@ -0,0 +1,43 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class CompletionTests
+{
+    [Fact]
+    public static void Completion_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new Completion
+        {
+            Values = ["option1", "option2", "option3"],
+            Total = 50,
+            HasMore = true
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Completion>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal(3, deserialized.Values.Count);
+        Assert.Equal("option1", deserialized.Values[0]);
+        Assert.Equal("option2", deserialized.Values[1]);
+        Assert.Equal("option3", deserialized.Values[2]);
+        Assert.Equal(50, deserialized.Total);
+        Assert.True(deserialized.HasMore);
+    }
+
+    [Fact]
+    public static void Completion_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new Completion();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Completion>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Empty(deserialized.Values);
+        Assert.Null(deserialized.Total);
+        Assert.Null(deserialized.HasMore);
+    }
+}