Final improvements: Extract constant, initialize fields, add null checks

- Extract JSON-RPC version "2.0" to JsonRpcVersion constant
- Initialize JsonRpc field in Union.Parse to avoid null warnings
- Set HasMethod/HasError flags only when values are non-null
- Add explicit null checks before constructing message objects
- Remove all null-forgiving operators for safer code
- All 1035 tests still pass

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

Author: Copilot

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -71,6 +71,8 @@ private protected JsonRpcMessage()
     [EditorBrowsable(EditorBrowsableState.Never)]
     public sealed class Converter : JsonConverter<JsonRpcMessage>
     {
+        private const string JsonRpcVersion = "2.0";
+
         /// <summary>
         /// Private struct to hold parsed JSON-RPC message data during deserialization.
         /// </summary>
@@ -102,7 +104,10 @@ private struct Union
             /// </summary>
             public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions options)
             {
-                var union = new Union();
+                var union = new Union
+                {
+                    JsonRpc = string.Empty // Initialize to avoid null reference warnings
+                };
 
                 if (reader.TokenType != JsonTokenType.StartObject)
                 {
@@ -142,7 +147,7 @@ public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions optio
 
                         case "method":
                             union.Method = reader.GetString();
-                            union.HasMethod = true;
+                            union.HasMethod = union.Method is not null;
                             break;
 
                         case "params":
@@ -151,7 +156,7 @@ public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions optio
 
                         case "error":
                             union.Error = JsonSerializer.Deserialize(ref reader, options.GetTypeInfo<JsonRpcErrorDetail>());
-                            union.HasError = true;
+                            union.HasError = union.Error is not null;
                             break;
 
                         case "result":
@@ -176,7 +181,7 @@ public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions optio
             var union = Union.Parse(ref reader, options);
 
             // All JSON-RPC messages must have a jsonrpc property with value "2.0"
-            if (union.JsonRpc != "2.0")
+            if (union.JsonRpc != JsonRpcVersion)
             {
                 throw new JsonException("Invalid or missing jsonrpc version");
             }
@@ -187,11 +192,16 @@ public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions optio
                 // Messages with an error property are error responses
                 if (union.HasError)
                 {
+                    if (union.Error is null)
+                    {
+                        throw new JsonException("Error property cannot be null");
+                    }
+
                     return new JsonRpcError
                     {
                         JsonRpc = union.JsonRpc,
                         Id = union.Id,
-                        Error = union.Error!
+                        Error = union.Error
                     };
                 }
 
@@ -212,22 +222,32 @@ public static Union Parse(ref Utf8JsonReader reader, JsonSerializerOptions optio
             // Messages with a method but no id are notifications
             if (union.HasMethod && !union.HasId)
             {
+                if (union.Method is null)
+                {
+                    throw new JsonException("Method property cannot be null");
+                }
+
                 return new JsonRpcNotification
                 {
                     JsonRpc = union.JsonRpc,
-                    Method = union.Method!,
+                    Method = union.Method,
                     Params = union.Params
                 };
             }
 
             // Messages with both method and id are requests
             if (union.HasMethod && union.HasId)
             {
+                if (union.Method is null)
+                {
+                    throw new JsonException("Method property cannot be null");
+                }
+
                 return new JsonRpcRequest
                 {
                     JsonRpc = union.JsonRpc,
                     Id = union.Id,
-                    Method = union.Method!,
+                    Method = union.Method,
                     Params = union.Params
                 };
             }