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>

tests-ui/tests/renderer/extensions/vueNodes/widgets/composables/useIntWidget.test.ts

@@ -1,72 +0,0 @@
-import { beforeEach, describe, expect, it, vi } from 'vitest'
-
-import { _for_testing } from '@/renderer/extensions/vueNodes/widgets/composables/useIntWidget'
-
-vi.mock('@/scripts/widgets', () => ({
-  addValueControlWidgets: vi.fn()
-}))
-
-vi.mock('@/platform/settings/settingStore', () => ({
-  useSettingStore: () => ({
-    settings: {}
-  })
-}))
-
-const { onValueChange } = _for_testing
-
-describe('useIntWidget', () => {
-  describe('onValueChange', () => {
-    let widget: any
-
-    beforeEach(() => {
-      // Reset the widget before each test
-      widget = {
-        options: {},
-        value: 0
-      }
-    })
-
-    it('should round values based on step size', () => {
-      widget.options.step2 = 0.1
-      onValueChange.call(widget, 5.7)
-      expect(widget.value).toBe(5.7)
-
-      widget.options.step2 = 0.5
-      onValueChange.call(widget, 7.3)
-      expect(widget.value).toBe(7.5)
-
-      widget.options.step2 = 1
-      onValueChange.call(widget, 23.4)
-      expect(widget.value).toBe(23)
-    })
-
-    it('should handle undefined step by using default of 1', () => {
-      widget.options.step2 = undefined
-      onValueChange.call(widget, 3.7)
-      expect(widget.value).toBe(4)
-    })
-
-    it('should account for min value offset', () => {
-      widget.options.step2 = 2
-      widget.options.min = 1
-      // 2 valid values between 1.6 are 1 and 3
-      // 1.6 is closer to 1, so it should round to 1
-      onValueChange.call(widget, 1.6)
-      expect(widget.value).toBe(1)
-    })
-
-    it('should handle undefined min by using default of 0', () => {
-      widget.options.step2 = 2
-      widget.options.min = undefined
-      onValueChange.call(widget, 5.7)
-      expect(widget.value).toBe(6)
-    })
-
-    it('should handle NaN shift value', () => {
-      widget.options.step2 = 0
-      widget.options.min = 1
-      onValueChange.call(widget, 5.7)
-      expect(widget.value).toBe(6)
-    })
-  })
-})