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/PromptsCapabilityTests.cs

@@ -0,0 +1,34 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class PromptsCapabilityTests
+{
+    [Fact]
+    public static void PromptsCapability_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new PromptsCapability
+        {
+            ListChanged = true
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<PromptsCapability>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.True(deserialized.ListChanged);
+    }
+
+    [Fact]
+    public static void PromptsCapability_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new PromptsCapability();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<PromptsCapability>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Null(deserialized.ListChanged);
+    }
+}