fix(tui): restore missing mock attribute assignments in tui_app_coverage_steps

The _build_mock_textual() function in tui_app_coverage_steps.py was missing the attribute assignments that wire mock classes to the mock module objects. Without these assignments, the app module's import gate could not find App, Screen, Vertical, Horizontal, Static, TextArea, or Collapsible on the mock modules, causing _TEXTUAL_AVAILABLE to remain False and all tui_app_coverage and tui_mainscreen_sidebar scenarios to fail with AttributeError.

Adds the 9 missing assignments:
  mock_textual_app.App = MockApp
  mock_textual_app.Screen = MockScreen
  mock_textual_containers.Vertical = MockVertical
  mock_textual_containers.Horizontal = MockHorizontal
  mock_textual_widgets.Header = MockHeader
  mock_textual_widgets.Footer = MockFooter
  mock_textual_widgets.Static = MockStatic
  mock_textual_widgets.TextArea = MockTextArea
  mock_textual_widgets.Collapsible = MockCollapsible

features/steps/tui_app_coverage_steps.py

@@ -126,6 +126,16 @@ def _build_mock_textual():
         def __exit__(self, *args):
             pass
 
+    mock_textual_app.App = MockApp
+    mock_textual_app.Screen = MockScreen
+    mock_textual_containers.Vertical = MockVertical
+    mock_textual_containers.Horizontal = MockHorizontal
+    mock_textual_widgets.Header = MockHeader
+    mock_textual_widgets.Footer = MockFooter
+    mock_textual_widgets.Static = MockStatic
+    mock_textual_widgets.TextArea = MockTextArea
+    mock_textual_widgets.Collapsible = MockCollapsible
+
     return {
         "textual": mock_textual,
         "textual.app": mock_textual_app,