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>

src/scripts/api.folderPaths.test.ts

@@ -0,0 +1,30 @@
+import axios from 'axios'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { api } from '@/scripts/api'
+
+vi.mock('axios')
+
+describe('getFolderPaths', () => {
+  beforeEach(() => {
+    vi.resetAllMocks()
+  })
+
+  it('returns legacy API response when available', async () => {
+    const mockResponse = { checkpoints: ['/test/checkpoints'] }
+    vi.mocked(axios.get).mockResolvedValueOnce({ data: mockResponse })
+
+    const result = await api.getFolderPaths()
+
+    expect(result).toEqual(mockResponse)
+  })
+
+  it('returns empty object when legacy API unavailable (dynamic discovery)', async () => {
+    vi.mocked(axios.get).mockRejectedValueOnce(new Error())
+
+    const result = await api.getFolderPaths()
+
+    // With dynamic discovery, we don't pre-generate directories when API is unavailable
+    expect(result).toEqual({})
+  })
+})