fix: reorder auto-datasource before child slots + validate Caption presence

- Move auto-datasource resolution (step 4) before applyChildSlots (step 4.1)
  so entityContext is available for child widgets that depend on it.
- Return validation error from setWidgetCaption when widget has no Caption
  property, matching setWidgetContent behavior (instead of silently injecting
  a plain string field).

Author: retran

mdl/executor/bson_helpers.go

@@ -356,8 +356,7 @@ func setRawWidgetProperty(widget bson.D, propName string, value interface{}) err
 func setWidgetCaption(widget bson.D, value interface{}) error {
 	caption := dGetDoc(widget, "Caption")
 	if caption == nil {
-		setTranslatableText(widget, "Caption", value)
-		return nil
+		return mdlerrors.NewValidation("widget has no Caption property")
 	}
 	setTranslatableText(caption, "", value)
 	return nil

mdl/executor/widget_engine.go

@@ -129,12 +129,9 @@ func (e *PluggableWidgetEngine) Build(def *WidgetDefinition, w *ast.WidgetV3) (*
 		}
 	}
 
-	// 4. Apply child slots (.def.json)
-	if err := e.applyChildSlots(builder, slots, w, propertyTypeIDs); err != nil {
-		return nil, err
-	}
-
-	// 4.1 Auto datasource: map AST DataSource to first DataSource-type property.
+	// 4. Auto datasource: map AST DataSource to first DataSource-type property.
+	// This must run before child slots so that entityContext is available
+	// for child widgets that depend on the parent's data source.
 	dsHandledByMapping := false
 	for _, m := range mappings {
 		if m.Source == "DataSource" {
@@ -160,6 +157,11 @@ func (e *PluggableWidgetEngine) Build(def *WidgetDefinition, w *ast.WidgetV3) (*
 		}
 	}
 
+	// 4.1 Apply child slots (.def.json)
+	if err := e.applyChildSlots(builder, slots, w, propertyTypeIDs); err != nil {
+		return nil, err
+	}
+
 	// 4.3 Auto child slots: match AST children to Widgets-type template properties.
 	handledSlotKeys := make(map[string]bool)
 	for _, s := range slots {