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/litegraph/utils/widget.test.ts

@@ -1,44 +0,0 @@
-import { describe, expect, test } from 'vitest'
-
-import type { IWidgetOptions } from '@/lib/litegraph/src/litegraph'
-import { getWidgetStep } from '@/lib/litegraph/src/litegraph'
-
-describe('getWidgetStep', () => {
-  test('should return step2 when available', () => {
-    const options: IWidgetOptions<unknown> = {
-      step2: 0.5,
-      step: 20
-    }
-
-    expect(getWidgetStep(options)).toBe(0.5)
-  })
-
-  test('should calculate from step when step2 is not available', () => {
-    const options: IWidgetOptions<unknown> = {
-      step: 20
-    }
-
-    expect(getWidgetStep(options)).toBe(2) // 20 * 0.1 = 2
-  })
-
-  test('should use default step value of 10 when neither step2 nor step is provided', () => {
-    const options: IWidgetOptions<unknown> = {}
-
-    expect(getWidgetStep(options)).toBe(1) // 10 * 0.1 = 1
-  })
-  // Zero value is not allowed for step, fallback to 1.
-  test('should handle zero values correctly', () => {
-    const optionsWithZeroStep2: IWidgetOptions<unknown> = {
-      step2: 0,
-      step: 20
-    }
-
-    expect(getWidgetStep(optionsWithZeroStep2)).toBe(2)
-
-    const optionsWithZeroStep: IWidgetOptions<unknown> = {
-      step: 0
-    }
-
-    expect(getWidgetStep(optionsWithZeroStep)).toBe(1)
-  })
-})