test: add dedicated component test for SlotContextMenu.vue (positioning, show/hide lifecycle, dialog integration)

## Summary

Add a dedicated component test for `SlotContextMenu.vue` to cover the complexity that lives in the component itself, beyond what the pure-function unit tests (`canRenameSlot`, `renameSlot`) and e2e tests already cover.

## Motivation

Currently, unit tests cover the pure service functions in `slotMenuService.ts` and e2e tests cover the full rename/disconnect/remove flow. However, there is no component-level test for `SlotContextMenu.vue` itself, where most of the complexity lives.

## Scope

A component test (e.g., `SlotContextMenu.test.ts`) should cover at minimum:

- **Positioning logic**: world-to-screen coordinate translation accounting for canvas scale and offset; `updateMenuPosition()` repositions correctly when canvas transform changes.
- **Show/hide lifecycle**: `show(event, context)` opens the menu and starts the RAF sync loop; `hide()` closes it and stops the loop; `isOpen` reactive state is consistent.
- **Menu item construction**: items reflect `canRenameSlot(ctx)` result (Rename item present/absent based on slot state).
- **Dialog integration**: `handleRename(ctx)` calls `dialogService.prompt()` and delegates to `renameSlot` on confirmation; no-ops on cancel.

## Related

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9045
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9045#discussion_r2925519412

/cc @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11893-test-add-dedicated-component-test-for-SlotContextMenu-vue-positioning-show-hide-li-3566d73d365081c6b3c7e4c6623f09a8) by [Unito](https://www.unito.io)