chore: migrate tests from tests-ui/ to colocate with source files (#7811)

## Summary

Migrates all unit tests from `tests-ui/` to colocate with their source
files in `src/`, improving discoverability and maintainability.

## Changes

- **What**: Relocated all unit tests to be adjacent to the code they
test, following the `<source>.test.ts` naming convention
- **Config**: Updated `vitest.config.ts` to remove `tests-ui` include
pattern and `@tests-ui` alias
- **Docs**: Moved testing documentation to `docs/testing/` with updated
paths and patterns

## Review Focus

- Migration patterns documented in
`temp/plans/migrate-tests-ui-to-src.md`
- Tests use `@/` path aliases instead of relative imports
- Shared fixtures placed in `__fixtures__/` directories

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7811-chore-migrate-tests-from-tests-ui-to-colocate-with-source-files-2da6d73d36508147a4cce85365dee614)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: GitHub Action <action@github.com>

src/renderer/extensions/vueNodes/widgets/composables/useAssetWidgetData.desktop.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it, vi } from 'vitest'
+import { ref } from 'vue'
+
+import { useAssetWidgetData } from '@/renderer/extensions/vueNodes/widgets/composables/useAssetWidgetData'
+
+vi.mock('@/platform/distribution/types', () => ({
+  isCloud: false
+}))
+
+const mockUpdateModelsForNodeType = vi.fn()
+const mockGetCategoryForNodeType = vi.fn()
+
+vi.mock('@/stores/assetsStore', () => ({
+  useAssetsStore: () => ({
+    modelAssetsByNodeType: new Map(),
+    modelLoadingByNodeType: new Map(),
+    modelErrorByNodeType: new Map(),
+    updateModelsForNodeType: mockUpdateModelsForNodeType
+  })
+}))
+
+vi.mock('@/stores/modelToNodeStore', () => ({
+  useModelToNodeStore: () => ({
+    getCategoryForNodeType: mockGetCategoryForNodeType
+  })
+}))
+
+describe('useAssetWidgetData (desktop/isCloud=false)', () => {
+  it('returns empty/default values without calling stores', () => {
+    const nodeType = ref('CheckpointLoaderSimple')
+    const { category, assets, dropdownItems, isLoading, error } =
+      useAssetWidgetData(nodeType)
+
+    expect(category.value).toBeUndefined()
+    expect(assets.value).toEqual([])
+    expect(dropdownItems.value).toEqual([])
+    expect(isLoading.value).toBe(false)
+    expect(error.value).toBeNull()
+    expect(mockUpdateModelsForNodeType).not.toHaveBeenCalled()
+    expect(mockGetCategoryForNodeType).not.toHaveBeenCalled()
+  })
+})