Address code review feedback: Add documentation and null validation

- Add XML documentation comments for Union struct fields
- Add null validation for property names
- Replace null-forgiving operators with defensive null handling
- Use `?? string.Empty` for jsonrpc property
- All 1035 tests still pass

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -76,15 +76,25 @@ public sealed class Converter : JsonConverter<JsonRpcMessage>
         /// </summary>
         private struct Union
         {
+            /// <summary>The JSON-RPC protocol version (must be "2.0").</summary>
             public string JsonRpc;
+            /// <summary>The message identifier for requests and responses.</summary>
             public RequestId Id;
+            /// <summary>The method name for requests and notifications.</summary>
             public string? Method;
+            /// <summary>The parameters for requests and notifications.</summary>
             public JsonNode? Params;
+            /// <summary>The error details for error responses.</summary>
             public JsonRpcErrorDetail? Error;
+            /// <summary>The result for successful responses.</summary>
             public JsonNode? Result;
+            /// <summary>Indicates whether an 'id' property was present.</summary>
             public bool HasId;
+            /// <summary>Indicates whether a 'method' property was present.</summary>
             public bool HasMethod;
+            /// <summary>Indicates whether an 'error' property was present.</summary>
             public bool HasError;
+            /// <summary>Indicates whether a 'result' property was present.</summary>
             public bool HasResult;
 
             /// <summary>
@@ -111,13 +121,18 @@ public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions optio
                         throw new JsonException("Expected PropertyName token");
                     }
 
-                    string propertyName = reader.GetString()!;
+                    string? propertyName = reader.GetString();
+                    if (propertyName is null)
+                    {
+                        throw new JsonException("Property name cannot be null");
+                    }
+
                     reader.Read();
 
                     switch (propertyName)
                     {
                         case "jsonrpc":
-                            union.JsonRpc = reader.GetString()!;
+                            union.JsonRpc = reader.GetString() ?? string.Empty;
                             break;
 
                         case "id":