test(ensemble): Page header storage timer dispose regression tests

modules/ensemble/test/widget/page_storage_timer_dispose_test.dart

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+import 'package:ensemble/framework/data_context.dart';
+import 'package:ensemble/framework/view/page.dart';
+import 'package:ensemble/page_model.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:yaml/yaml.dart';
+
+/// Regression coverage for [PageState] dispose: periodic timers and storage
+/// subscriptions created for reactive header height / collapsible visibility
+/// must be cancelled so callbacks never call [setState] after dispose.
+void main() {
+  testWidgets(
+    'Page cancels title-bar height poll timer and storage listener on dispose',
+    (tester) async {
+      final doc = loadYaml(
+        '''
+View:
+  body:
+    Column:
+      children:
+        - Text:
+            text: body
+  header:
+    titleText: Title
+    styles:
+      listenTitleBarHeightStorage: true
+      titleBarHeight: ensemble.storage.tbHeight
+''',
+      ) as YamlMap;
+
+      final model = PageModel.fromYaml(doc) as SinglePageModel;
+      expect(model.headerModel, isNotNull);
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Builder(
+            builder: (context) => Page(
+              dataContext: DataContext(buildContext: context),
+              pageModel: model,
+              onRendered: () {},
+            ),
+          ),
+        ),
+      );
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 50));
+
+      await tester.pumpWidget(const SizedBox.shrink());
+      await tester.pump();
+
+      for (var i = 0; i < 25; i++) {
+        await tester.pump(const Duration(milliseconds: 100));
+      }
+
+      expect(tester.takeException(), isNull);
+    },
+  );
+
+  testWidgets(
+    'Page cancels collapsible header visibility poll timer on dispose',
+    (tester) async {
+      final doc = loadYaml(
+        '''
+View:
+  body:
+    Column:
+      children:
+        - Text:
+            text: body
+  header:
+    titleText: Title
+    styles:
+      collapsibleHeader:
+        enabled: true
+        visible: ensemble.storage.hdrVis
+''',
+      ) as YamlMap;
+
+      final model = PageModel.fromYaml(doc) as SinglePageModel;
+      expect(model.headerModel, isNotNull);
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Builder(
+            builder: (context) => Page(
+              dataContext: DataContext(buildContext: context),
+              pageModel: model,
+              onRendered: () {},
+            ),
+          ),
+        ),
+      );
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 50));
+
+      await tester.pumpWidget(const SizedBox.shrink());
+      await tester.pump();
+
+      for (var i = 0; i < 25; i++) {
+        await tester.pump(const Duration(milliseconds: 100));
+      }
+
+      expect(tester.takeException(), isNull);
+    },
+  );
+}