test: App mode - setting widget value test (#10746)

## Summary

Adds a test for setting various types of widgets in app mode, then
validating the /prompt API is called with the expected values

## Changes

- **What**: 
- extract duplicated enableLinearMode
- add AppModeWidgetHelper for setting values

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10746-test-App-mode-setting-widget-value-test-3336d73d365081739598fb5280d0127e)
by [Unito](https://www.unito.io)

browser_tests/tests/appModeWidgetRename.spec.ts

@@ -9,13 +9,7 @@ import {
 
 test.describe('App mode widget rename', { tag: ['@ui', '@subgraph'] }, () => {
   test.beforeEach(async ({ comfyPage }) => {
-    await comfyPage.page.evaluate(() => {
-      window.app!.api.serverFeatureFlags.value = {
-        ...window.app!.api.serverFeatureFlags.value,
-        linear_toggle_enabled: true
-      }
-    })
-    await comfyPage.settings.setSetting('Comfy.UseNewMenu', 'Top')
+    await comfyPage.appMode.enableLinearMode()
     await comfyPage.settings.setSetting(
       'Comfy.AppBuilder.VueNodeSwitchDismissed',
       true