add function to apply mutations over grpc

pkg/actor/mutation_registry.go

@@ -212,9 +212,9 @@ func (r *ProtoMutationRegistry) Apply(ctx context.Context, grain any, msg ...pro
 	}
 
 	for _, m := range msg {
-		// Ignore nil mutation elements (untyped or typed nil pointers) silently; they carry no data.
+		// Error if any mutation element is nil.
 		if m == nil {
-			continue
+			return results, fmt.Errorf("nil mutation message")
 		}
 		// Typed nil: interface holds concrete proto message type whose pointer value is nil.
 		rv := reflect.ValueOf(m)
@@ -251,6 +251,12 @@ func (r *ProtoMutationRegistry) Apply(ctx context.Context, grain any, msg ...pro
 			}
 		}
 	}
+	// Return error for unregistered mutations
+	for _, res := range results {
+		if res.Error == ErrMutationNotRegistered {
+			return results, res.Error
+		}
+	}
 	return results, nil
 }