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/NodeSlot.test.ts

@@ -1,83 +0,0 @@
-import { describe, expect, it } from 'vitest'
-
-import type {
-  INodeInputSlot,
-  INodeOutputSlot
-} from '@/lib/litegraph/src/litegraph'
-import {
-  inputAsSerialisable,
-  outputAsSerialisable
-} from '@/lib/litegraph/src/litegraph'
-
-describe('NodeSlot', () => {
-  describe('inputAsSerialisable', () => {
-    it('removes _data from serialized slot', () => {
-      // @ts-expect-error Missing boundingRect property for test
-      const slot: INodeOutputSlot = {
-        _data: 'test data',
-        name: 'test-id',
-        type: 'STRING',
-        links: []
-      }
-      // @ts-expect-error Argument type mismatch for test
-      const serialized = outputAsSerialisable(slot)
-      expect(serialized).not.toHaveProperty('_data')
-    })
-
-    it('removes pos from widget input slots', () => {
-      const widgetInputSlot: INodeInputSlot = {
-        name: 'test-id',
-        pos: [10, 20],
-        type: 'STRING',
-        link: null,
-        widget: {
-          name: 'test-widget',
-          // @ts-expect-error TODO: Fix after merge - type property not in IWidgetLocator
-          type: 'combo',
-          value: 'test-value-1',
-          options: {
-            values: ['test-value-1', 'test-value-2']
-          }
-        }
-      }
-
-      const serialized = inputAsSerialisable(widgetInputSlot)
-      expect(serialized).not.toHaveProperty('pos')
-    })
-
-    it('preserves pos for non-widget input slots', () => {
-      // @ts-expect-error TODO: Fix after merge - missing boundingRect property for test
-      const normalSlot: INodeInputSlot = {
-        name: 'test-id',
-        type: 'STRING',
-        pos: [10, 20],
-        link: null
-      }
-      const serialized = inputAsSerialisable(normalSlot)
-      expect(serialized).toHaveProperty('pos')
-    })
-
-    it('preserves only widget name during serialization', () => {
-      const widgetInputSlot: INodeInputSlot = {
-        name: 'test-id',
-        type: 'STRING',
-        link: null,
-        widget: {
-          name: 'test-widget',
-          // @ts-expect-error TODO: Fix after merge - type property not in IWidgetLocator
-          type: 'combo',
-          value: 'test-value-1',
-          options: {
-            values: ['test-value-1', 'test-value-2']
-          }
-        }
-      }
-
-      const serialized = inputAsSerialisable(widgetInputSlot)
-      expect(serialized.widget).toEqual({ name: 'test-widget' })
-      expect(serialized.widget).not.toHaveProperty('type')
-      expect(serialized.widget).not.toHaveProperty('value')
-      expect(serialized.widget).not.toHaveProperty('options')
-    })
-  })
-})