test: document empty change refresh validity

Symptom: the empty change-object fix relies on RefreshInClient=true even though the CE0032 text mentions only missing items and commit state.

Root cause: the Studio Pro validator accepts refresh-only empty changes, but that behavior is not obvious from public docs or the CE0032 wording.

Fix: add an executable bug-test fixture for issue #339 and document the mx check behavior directly beside the builder inference.

Tests: `./bin/mxcli check mdl-examples/bug-tests/339-empty-change-refresh-in-client.mdl` and `go test ./mdl/executor -run TestEmptyChangeObjectRefreshesInClient`.

Author: hjothamendix

mdl-examples/bug-tests/339-empty-change-refresh-in-client.mdl

@@ -0,0 +1,33 @@
+-- ============================================================================
+-- Bug #339: Empty change-object actions must refresh in client
+-- ============================================================================
+--
+-- Symptom (before fix):
+--   Rebuilding an empty change-object activity from MDL wrote a non-committing
+--   ChangeObjectAction with no member changes and RefreshInClient=false.
+--   Studio Pro / mx check rejected that activity with CE0032:
+--     "Change action has no items specified and does not commit the object."
+--
+-- After fix:
+--   `change $Object;` is serialized as a refresh-only change action by setting
+--   RefreshInClient=true. Although the CE0032 text mentions only items/commit,
+--   empirical mx check validation accepts refresh as the third valid escape.
+--
+-- Usage:
+--   mxcli exec mdl-examples/bug-tests/339-empty-change-refresh-in-client.mdl -p app.mpr
+--   mx check app.mpr
+--   Expected: 0 new CE0032 errors for BugTest339.MF_RefreshOnlyChange.
+-- ============================================================================
+
+create module BugTest339;
+
+create entity BugTest339.Item (
+  Name: String(100)
+);
+
+create microflow BugTest339.MF_RefreshOnlyChange ()
+begin
+  $Item = create BugTest339.Item;
+  change $Item;
+end;
+/

mdl/executor/cmd_microflows_builder_actions.go

@@ -240,9 +240,12 @@ func (fb *flowBuilder) addRollbackAction(s *ast.RollbackStmt) model.ID {
 // addChangeObjectAction creates a CHANGE statement.
 func (fb *flowBuilder) addChangeObjectAction(s *ast.ChangeObjectStmt) model.ID {
 	action := &microflows.ChangeObjectAction{
-		BaseElement:     model.BaseElement{ID: model.ID(types.GenerateID())},
-		ChangeVariable:  s.Variable,
-		Commit:          microflows.CommitTypeNo,
+		BaseElement:    model.BaseElement{ID: model.ID(types.GenerateID())},
+		ChangeVariable: s.Variable,
+		Commit:         microflows.CommitTypeNo,
+		// Studio Pro rejects an empty non-committing change action unless it
+		// refreshes in client. The CE0032 message mentions only items/commit,
+		// but mx check accepts RefreshInClient=true as the third valid escape.
 		RefreshInClient: len(s.Changes) == 0,
 	}