Address code review feedback

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -771,8 +771,11 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>
 
     /// <summary>
     /// Converts the Exception.Data dictionary to a serializable Dictionary&lt;string, object?&gt;.
-    /// Returns null if the data dictionary is empty.
+    /// Returns null if the data dictionary is empty or contains no string keys.
     /// </summary>
+    /// <remarks>
+    /// Only entries with string keys are included in the result. Entries with non-string keys are ignored.
+    /// </remarks>
     private static Dictionary<string, object?>? ConvertExceptionData(System.Collections.IDictionary data)
     {
         if (data.Count == 0)

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -721,11 +721,9 @@ await transport.SendMessageAsync(
         Assert.NotNull(error.Error.Data);
 
         // Verify the data contains the uri
-        var dataJson = JsonSerializer.Serialize(error.Error.Data, McpJsonUtilities.DefaultOptions);
-        var dataObject = JsonSerializer.Deserialize<JsonObject>(dataJson, McpJsonUtilities.DefaultOptions);
-        Assert.NotNull(dataObject);
-        Assert.True(dataObject.ContainsKey("uri"));
-        Assert.Equal(resourceUri, dataObject["uri"]?.GetValue<string>());
+        var dataDict = Assert.IsType<Dictionary<string, object?>>(error.Error.Data);
+        Assert.True(dataDict.ContainsKey("uri"));
+        Assert.Equal(resourceUri, dataDict["uri"]);
 
         await transport.DisposeAsync();
         await runTask;