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/GetPromptRequestParamsTests.cs

@@ -0,0 +1,49 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class GetPromptRequestParamsTests
+{
+    [Fact]
+    public static void GetPromptRequestParams_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new GetPromptRequestParams
+        {
+            Name = "code_review",
+            Arguments = new Dictionary<string, JsonElement>
+            {
+                ["language"] = JsonDocument.Parse("\"csharp\"").RootElement.Clone()
+            },
+            Meta = new JsonObject { ["progressToken"] = "tok-1" }
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<GetPromptRequestParams>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("code_review", deserialized.Name);
+        Assert.NotNull(deserialized.Arguments);
+        Assert.Equal("csharp", deserialized.Arguments["language"].GetString());
+        Assert.NotNull(deserialized.Meta);
+        Assert.Equal("tok-1", (string)deserialized.Meta["progressToken"]!);
+    }
+
+    [Fact]
+    public static void GetPromptRequestParams_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new GetPromptRequestParams
+        {
+            Name = "simple_prompt"
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<GetPromptRequestParams>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal("simple_prompt", deserialized.Name);
+        Assert.Null(deserialized.Arguments);
+        Assert.Null(deserialized.Meta);
+    }
+}