test: add E2E test for subgraph duplicate independent widget values (#10949)

## Summary
- Add E2E test verifying duplicated subgraphs maintain independent
widget values (convert CLIP node to subgraph, duplicate, set different
text in each, assert no bleed)
- Extract `clickMenuItemExact` and `openForVueNode` into `ContextMenu`
fixture for reuse across Vue node tests
- Refactor `contextMenu.spec.ts` to delegate to the new fixture methods

## Test plan
- [x] `pnpm typecheck:browser` passes
- [x] `pnpm lint` passes
- [x] New test passes locally (`pnpm test:browser:local --
browser_tests/tests/subgraph/subgraphDuplicateIndependentValues.spec.ts`)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10949-test-add-E2E-test-for-subgraph-duplicate-independent-widget-values-33b6d73d3650818191c1f78ef8db4455)
by [Unito](https://www.unito.io)

browser_tests/tests/vueNodes/interactions/node/contextMenu.spec.ts

@@ -10,17 +10,14 @@ import { TestIds } from '../../../../fixtures/selectors'
 const BYPASS_CLASS = /before:bg-bypass\/60/
 
 async function clickExactMenuItem(comfyPage: ComfyPage, name: string) {
-  await comfyPage.page.getByRole('menuitem', { name, exact: true }).click()
+  await comfyPage.contextMenu.clickMenuItemExact(name)
   await comfyPage.nextFrame()
 }
 
 async function openContextMenu(comfyPage: ComfyPage, nodeTitle: string) {
   const fixture = await comfyPage.vueNodes.getFixtureByTitle(nodeTitle)
-  await fixture.header.click()
-  await fixture.header.click({ button: 'right' })
-  const menu = comfyPage.contextMenu.primeVueMenu
-  await menu.waitFor({ state: 'visible' })
-  return menu
+  await comfyPage.contextMenu.openForVueNode(fixture.header)
+  return comfyPage.contextMenu.primeVueMenu
 }
 
 async function openMultiNodeContextMenu(