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/AnnotationsTests.cs

@@ -0,0 +1,43 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class AnnotationsTests
+{
+    [Fact]
+    public static void Annotations_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new Annotations
+        {
+            Audience = [Role.User, Role.Assistant],
+            Priority = 0.75f,
+            LastModified = new DateTimeOffset(2025, 6, 15, 10, 30, 0, TimeSpan.Zero)
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Annotations>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Audience);
+        Assert.Equal(2, deserialized.Audience.Count);
+        Assert.Equal(Role.User, deserialized.Audience[0]);
+        Assert.Equal(Role.Assistant, deserialized.Audience[1]);
+        Assert.Equal(original.Priority, deserialized.Priority);
+        Assert.Equal(original.LastModified, deserialized.LastModified);
+    }
+
+    [Fact]
+    public static void Annotations_SerializationRoundTrip_WithMinimalProperties()
+    {
+        var original = new Annotations();
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<Annotations>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Null(deserialized.Audience);
+        Assert.Null(deserialized.Priority);
+        Assert.Null(deserialized.LastModified);
+    }
+}