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/CompleteRequestParamsTests.cs

@@ -0,0 +1,58 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class CompleteRequestParamsTests
+{
+    [Fact]
+    public static void CompleteRequestParams_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new CompleteRequestParams
+        {
+            Ref = new PromptReference { Name = "my_prompt" },
+            Argument = new Argument { Name = "topic", Value = "wea" },
+            Context = new CompleteContext
+            {
+                Arguments = new Dictionary<string, string> { ["language"] = "en" }
+            },
+            Meta = new JsonObject { ["progressToken"] = "tok-1" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CompleteRequestParams>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        var promptRef = Assert.IsType<PromptReference>(deserialized.Ref);
+        Assert.Equal("my_prompt", promptRef.Name);
+        Assert.Equal(original.Argument.Name, deserialized.Argument.Name);
+        Assert.Equal(original.Argument.Value, deserialized.Argument.Value);
+        Assert.NotNull(deserialized.Context);
+        Assert.NotNull(deserialized.Context.Arguments);
+        Assert.Equal("en", deserialized.Context.Arguments["language"]);
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("tok-1", (string)deserialized.Meta["progressToken"]!);
+    }
+
+    [Fact]
+    public static void CompleteRequestParams_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new CompleteRequestParams
+        {
+            Ref = new ResourceTemplateReference { Uri = "file:///{path}" },
+            Argument = new Argument { Name = "path", Value = "/ho" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<CompleteRequestParams>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        var resourceRef = Assert.IsType<ResourceTemplateReference>(deserialized.Ref);
+        Assert.Equal("file:///{path}", resourceRef.Uri);
+        Assert.Equal("path", deserialized.Argument.Name);
+        Assert.Equal("/ho", deserialized.Argument.Value);
+        Assert.Null(deserialized.Context);
+        Assert.Null(deserialized.Meta);
+    }
+}