Fix JsonRpc serialization of args and formatting

SerializeArgs was calling GetTypeInfo(typeof(object?[])), which is not provided by any of the source-generated JsonSerializerContexts and therefore threw NotSupportedException at runtime on macOS/Ubuntu (Windows happened to skip these code paths in earlier failures). Build the params JSON array manually, looking up TypeInfo by each argument's runtime type, which is what the merged source-gen resolver actually contains.

Also insert the missing space before ':' in the PendingRequest primary-constructor base list to satisfy 'dotnet format --verify-no-changes'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/src/JsonRpc.cs

@@ -565,7 +565,31 @@ private async Task HandleIncomingMethodAsync(string methodName, JsonElement mess
             return null;
         }
 
-        return JsonSerializer.SerializeToElement(args, _serializerOptions.GetTypeInfo(typeof(object?[])));
+        // Source-generated JsonSerializerOptions do not provide metadata for object[],
+        // so build the JSON array manually, serializing each element with a TypeInfo
+        // looked up by its runtime type from the merged resolver.
+        var buffer = new ArrayBufferWriter<byte>();
+        using (var writer = new Utf8JsonWriter(buffer))
+        {
+            writer.WriteStartArray();
+            foreach (var arg in args)
+            {
+                if (arg is null)
+                {
+                    writer.WriteNullValue();
+                }
+                else
+                {
+                    var typeInfo = _serializerOptions.GetTypeInfo(arg.GetType());
+                    JsonSerializer.Serialize(writer, arg, typeInfo);
+                }
+            }
+
+            writer.WriteEndArray();
+        }
+
+        using var doc = JsonDocument.Parse(buffer.WrittenMemory);
+        return doc.RootElement.Clone();
     }
 
     private async Task SendResultResponseAsync(JsonElement id, object? result, CancellationToken cancellationToken)
@@ -618,7 +642,7 @@ await SendMessageAsync(new JsonRpcNotification
         }
     }
 
-    private sealed class PendingRequest(): TaskCompletionSource<JsonElement>(TaskCreationOptions.RunContinuationsAsynchronously);
+    private sealed class PendingRequest() : TaskCompletionSource<JsonElement>(TaskCreationOptions.RunContinuationsAsynchronously);
 
     private sealed class MethodRegistration
     {