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/core/LLink.test.ts

@@ -1,17 +0,0 @@
-import { describe, expect } from 'vitest'
-
-import { LLink } from '@/lib/litegraph/src/litegraph'
-
-import { test } from './fixtures/testExtensions'
-
-describe('LLink', () => {
-  test('matches previous snapshot', () => {
-    const link = new LLink(1, 'float', 4, 2, 5, 3)
-    expect(link.serialize()).toMatchSnapshot('Basic')
-  })
-
-  test('serializes to the previous snapshot', () => {
-    const link = new LLink(1, 'float', 4, 2, 5, 3)
-    expect(link.serialize()).toMatchSnapshot('Basic')
-  })
-})