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/utils/migration/migrateReroute.test.ts

@@ -1,37 +0,0 @@
-import fs from 'node:fs'
-import path from 'node:path'
-import { describe, expect, it } from 'vitest'
-
-import type { WorkflowJSON04 } from '@/platform/workflow/validation/schemas/workflowSchema'
-import { migrateLegacyRerouteNodes } from '@/utils/migration/migrateReroute'
-
-describe('migrateReroute', () => {
-  describe('migrateReroute snapshots', () => {
-    // Helper function to load workflow JSON files
-    const loadWorkflow = (filePath: string): WorkflowJSON04 => {
-      const fullPath = path.resolve(__dirname, filePath)
-      const fileContent = fs.readFileSync(fullPath, 'utf-8')
-      return JSON.parse(fileContent) as WorkflowJSON04
-    }
-
-    it.each([
-      'branching.json',
-      'single_connected.json',
-      'floating.json',
-      'floating_branch.json'
-    ])('should correctly migrate %s', async (fileName) => {
-      // Load the legacy workflow
-      const legacyWorkflow = loadWorkflow(
-        `workflows/reroute/legacy/${fileName}`
-      )
-
-      // Migrate the workflow
-      const migratedWorkflow = migrateLegacyRerouteNodes(legacyWorkflow)
-
-      // Compare with snapshot
-      await expect(
-        JSON.stringify(migratedWorkflow, null, 2)
-      ).toMatchFileSnapshot(`workflows/reroute/native/${fileName}`)
-    })
-  })
-})