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/UrlElicitationRequiredErrorDataTests.cs

@@ -0,0 +1,43 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class UrlElicitationRequiredErrorDataTests
+{
+    [Fact]
+    public static void UrlElicitationRequiredErrorData_SerializationRoundTrip_PreservesAllProperties()
+    {
+        var original = new UrlElicitationRequiredErrorData
+        {
+            Elicitations =
+            [
+                new ElicitRequestParams
+                {
+                    Mode = "url",
+                    ElicitationId = "elicit-1",
+                    Url = "https://example.com/auth",
+                    Message = "Please authenticate"
+                },
+                new ElicitRequestParams
+                {
+                    Mode = "url",
+                    ElicitationId = "elicit-2",
+                    Url = "https://example.com/consent",
+                    Message = "Please consent"
+                }
+            ]
+        };
+
+        string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<UrlElicitationRequiredErrorData>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.Equal(2, deserialized.Elicitations.Count);
+        Assert.Equal("url", deserialized.Elicitations[0].Mode);
+        Assert.Equal("elicit-1", deserialized.Elicitations[0].ElicitationId);
+        Assert.Equal("https://example.com/auth", deserialized.Elicitations[0].Url);
+        Assert.Equal("Please authenticate", deserialized.Elicitations[0].Message);
+        Assert.Equal("elicit-2", deserialized.Elicitations[1].ElicitationId);
+    }
+}