add function to apply mutations over grpc

pkg/cart/mutation_test.go

@@ -267,10 +267,10 @@ func TestMutationRegistryCoverage(t *testing.T) {
 		t.Fatalf("GetTypeName failed for AddItem, got (%q,%v)", nm, ok)
 	}
 
-	// Apply unregistered message -> result should contain ErrMutationNotRegistered, no top-level error
+	// Apply unregistered message -> should return error
 	results, err := reg.Apply(context.Background(), newTestGrain(), &messages.Noop{})
-	if err != nil {
-		t.Fatalf("unexpected top-level error applying unregistered mutation: %v", err)
+	if err == nil {
+		t.Fatalf("expected error for unregistered mutation")
 	}
 	if len(results) != 1 || results[0].Error == nil || results[0].Error != actor.ErrMutationNotRegistered {
 		t.Fatalf("expected ApplyResult with ErrMutationNotRegistered, got %#v", results)