Playwright tests should use user-accessible actions to open the Subgraph Editor (not internal commands)

## Summary

Playwright end-to-end tests should interact with the Subgraph Editor through user-accessible UI actions (e.g., clicking a button) rather than invoking internal commands such as `Comfy.Graph.EditSubgraphWidgets`.

## Context

In `browser_tests/fixtures/components/SubgraphEditor.ts`, the `open()` method currently uses an internal command to open the subgraph editor panel:

```ts
// TODO: don't use commands for this
await this.comfyPage.executeCommand('Comfy.Graph.EditSubgraphWidgets')
```

Using internal commands bypasses the real user interaction path (e.g., clicking a visible button in the UI), which may mask issues that real users could encounter.

## Desired Outcome

Identify and use a user-accessible UI action (e.g., a button click in the properties panel or node context menu) to open the Subgraph Editor in Playwright tests, replacing the `executeCommand` call.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11806
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11806#discussion_r3223979202

/cc @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-12176-Playwright-tests-should-use-user-accessible-actions-to-open-the-Subgraph-Editor-not--35e6d73d365081878507e62e661c95b6) by [Unito](https://www.unito.io)