refactor: extract composables from VTU holdout components, complete VTL migration (#10966)

## Summary

Extract internal logic from the 2 remaining VTU holdout components into
composables, enabling full VTL migration.

## Changes

- **What**: Extract `useProcessedWidgets` from `NodeWidgets.vue`
(486→135 LOC) and `useWidgetSelectItems`/`useWidgetSelectActions` from
`WidgetSelectDropdown.vue` (563→170 LOC). Rewrite both component test
files as composable unit tests + slim behavioral VTL tests. Remove
`@vue/test-utils` devDependency.
- **Dependencies**: Removes `@vue/test-utils`

## Review Focus

- Composable extraction is mechanical — no logic changes, just moving
code into testable units
- `useProcessedWidgets` handles widget deduplication, promotion border
styling, error detection, and identity resolution (~290 LOC)
- `useWidgetSelectItems` handles the full computed chain from widget
values → dropdown items including cloud asset mode and multi-output job
resolution (~350 LOC)
- `useWidgetSelectActions` handles selection resolution and file upload
(~120 LOC)
- 40 new composable-level unit tests replace 13 `wrapper.vm.*` accesses
across the 2 holdout files

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10966-refactor-extract-composables-from-VTU-holdout-components-complete-VTL-migration-33c6d73d36508148a3a4ccf346722d6d)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

docs/testing/component-testing.md

@@ -1,5 +1,7 @@
 # Component Testing Guide
 
+> **Note**: New component tests must use `@testing-library/vue` with `@testing-library/user-event`. The examples below that use `@vue/test-utils` (`mount`, `wrapper`) are from legacy tests. An ESLint rule enforces this — importing from `@vue/test-utils` in `*.test.ts` files produces a lint error.
+
 This guide covers patterns and examples for testing Vue components in the ComfyUI Frontend codebase.
 
 ## Table of Contents