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/components/TopMenuSection.test.ts

@@ -0,0 +1,103 @@
+import { createTestingPinia } from '@pinia/testing'
+import { mount } from '@vue/test-utils'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { computed } from 'vue'
+import { createI18n } from 'vue-i18n'
+
+import TopMenuSection from '@/components/TopMenuSection.vue'
+import CurrentUserButton from '@/components/topbar/CurrentUserButton.vue'
+import LoginButton from '@/components/topbar/LoginButton.vue'
+import { isElectron } from '@/utils/envUtil'
+
+const mockData = vi.hoisted(() => ({ isLoggedIn: false }))
+
+vi.mock('@/composables/auth/useCurrentUser', () => ({
+  useCurrentUser: () => {
+    return {
+      isLoggedIn: computed(() => mockData.isLoggedIn)
+    }
+  }
+}))
+
+vi.mock('@/utils/envUtil')
+vi.mock('@/stores/firebaseAuthStore', () => ({
+  useFirebaseAuthStore: vi.fn(() => ({
+    currentUser: null,
+    loading: false
+  }))
+}))
+
+function createWrapper() {
+  const i18n = createI18n({
+    legacy: false,
+    locale: 'en',
+    messages: {
+      en: {
+        sideToolbar: {
+          queueProgressOverlay: {
+            viewJobHistory: 'View job history',
+            expandCollapsedQueue: 'Expand collapsed queue'
+          }
+        }
+      }
+    }
+  })
+
+  return mount(TopMenuSection, {
+    global: {
+      plugins: [createTestingPinia({ createSpy: vi.fn }), i18n],
+      stubs: {
+        SubgraphBreadcrumb: true,
+        QueueProgressOverlay: true,
+        CurrentUserButton: true,
+        LoginButton: true
+      },
+      directives: {
+        tooltip: () => {}
+      }
+    }
+  })
+}
+
+describe('TopMenuSection', () => {
+  beforeEach(() => {
+    vi.resetAllMocks()
+  })
+
+  describe('authentication state', () => {
+    describe('when user is logged in', () => {
+      beforeEach(() => {
+        mockData.isLoggedIn = true
+      })
+
+      it('should display CurrentUserButton and not display LoginButton', () => {
+        const wrapper = createWrapper()
+        expect(wrapper.findComponent(CurrentUserButton).exists()).toBe(true)
+        expect(wrapper.findComponent(LoginButton).exists()).toBe(false)
+      })
+    })
+
+    describe('when user is not logged in', () => {
+      beforeEach(() => {
+        mockData.isLoggedIn = false
+      })
+
+      describe('on desktop platform', () => {
+        it('should display LoginButton and not display CurrentUserButton', () => {
+          vi.mocked(isElectron).mockReturnValue(true)
+          const wrapper = createWrapper()
+          expect(wrapper.findComponent(LoginButton).exists()).toBe(true)
+          expect(wrapper.findComponent(CurrentUserButton).exists()).toBe(false)
+        })
+      })
+
+      describe('on web platform', () => {
+        it('should not display CurrentUserButton and not display LoginButton', () => {
+          const wrapper = createWrapper()
+          expect(wrapper.findComponent(CurrentUserButton).exists()).toBe(false)
+          expect(wrapper.findComponent(LoginButton).exists()).toBe(false)
+        })
+      })
+    })
+  })
+})