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/ConfigureGraph.test.ts

@@ -1,21 +0,0 @@
-// TODO: Fix these tests after migration
-import { describe } from 'vitest'
-
-import { LGraph } from '@/lib/litegraph/src/litegraph'
-
-import { dirtyTest } from './fixtures/testExtensions'
-
-describe.skip('LGraph configure()', () => {
-  dirtyTest(
-    'LGraph matches previous snapshot (normal configure() usage)',
-    ({ expect, minimalSerialisableGraph, basicSerialisableGraph }) => {
-      const configuredMinGraph = new LGraph()
-      configuredMinGraph.configure(minimalSerialisableGraph)
-      expect(configuredMinGraph).toMatchSnapshot('configuredMinGraph')
-
-      const configuredBasicGraph = new LGraph()
-      configuredBasicGraph.configure(basicSerialisableGraph)
-      expect(configuredBasicGraph).toMatchSnapshot('configuredBasicGraph')
-    }
-  )
-})