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/platform/telemetry/useTelemetry.test.ts

@@ -1,30 +0,0 @@
-import { beforeEach, describe, expect, it, vi } from 'vitest'
-
-vi.mock('@/platform/distribution/types', () => ({
-  isCloud: false
-}))
-
-describe('useTelemetry', () => {
-  beforeEach(() => {
-    vi.clearAllMocks()
-  })
-
-  it('should return null when not in cloud distribution', async () => {
-    const { useTelemetry } = await import('@/platform/telemetry')
-    const provider = useTelemetry()
-
-    // Should return null for OSS builds
-    expect(provider).toBeNull()
-  }, 10000)
-
-  it('should return null consistently for OSS builds', async () => {
-    const { useTelemetry } = await import('@/platform/telemetry')
-
-    const provider1 = useTelemetry()
-    const provider2 = useTelemetry()
-
-    // Both should be null for OSS builds
-    expect(provider1).toBeNull()
-    expect(provider2).toBeNull()
-  })
-})