[backport cloud/1.36] feat: add polling fallback for stale asset downloads (#7981)

Backport of #7926 to cloud/1.36

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7981-backport-cloud-1-36-feat-add-polling-fallback-for-stale-asset-downloads-2e76d73d365081a983a4e5a8683ae2c9)
by [Unito](https://www.unito.io)

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

tests-ui/vitest-patterns.md

@@ -0,0 +1,138 @@
+---
+globs:
+  - '**/*.test.ts'
+  - '**/*.spec.ts'
+---
+
+# Vitest Patterns
+
+## Setup
+
+Use `createTestingPinia` from `@pinia/testing`, not `createPinia`:
+
+```typescript
+import { createTestingPinia } from '@pinia/testing'
+import { setActivePinia } from 'pinia'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+describe('MyStore', () => {
+  beforeEach(() => {
+    setActivePinia(createTestingPinia({ stubActions: false }))
+    vi.useFakeTimers()
+    vi.resetAllMocks()
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+})
+```
+
+**Why `stubActions: false`?** By default, testing pinia stubs all actions. Set to `false` when testing actual store behavior.
+
+## Mock Patterns
+
+### Reset all mocks at once
+
+```typescript
+beforeEach(() => {
+  vi.resetAllMocks()  // Not individual mock.mockReset() calls
+})
+```
+
+### Module mocks with vi.mock()
+
+```typescript
+vi.mock('@/scripts/api', () => ({
+  api: {
+    addEventListener: vi.fn(),
+    fetchData: vi.fn()
+  }
+}))
+
+vi.mock('@/services/myService', () => ({
+  myService: {
+    doThing: vi.fn()
+  }
+}))
+```
+
+### Configure mocks in tests
+
+```typescript
+import { api } from '@/scripts/api'
+import { myService } from '@/services/myService'
+
+it('handles success', () => {
+  vi.mocked(myService.doThing).mockResolvedValue({ data: 'test' })
+  // ... test code
+})
+```
+
+## Testing Event Listeners
+
+When a store registers event listeners at module load time:
+
+```typescript
+function getEventHandler() {
+  const call = vi.mocked(api.addEventListener).mock.calls.find(
+    ([event]) => event === 'my_event'
+  )
+  return call?.[1] as (e: CustomEvent<MyEventType>) => void
+}
+
+function dispatch(data: MyEventType) {
+  const handler = getEventHandler()
+  handler(new CustomEvent('my_event', { detail: data }))
+}
+
+it('handles events', () => {
+  const store = useMyStore()
+  dispatch({ field: 'value' })
+  expect(store.items).toHaveLength(1)
+})
+```
+
+## Testing with Fake Timers
+
+For stores with intervals, timeouts, or polling:
+
+```typescript
+beforeEach(() => {
+  vi.useFakeTimers()
+})
+
+afterEach(() => {
+  vi.useRealTimers()
+})
+
+it('polls after delay', async () => {
+  const store = useMyStore()
+  store.startPolling()
+
+  await vi.advanceTimersByTimeAsync(30000)
+
+  expect(mockService.fetch).toHaveBeenCalled()
+})
+```
+
+## Assertion Style
+
+Prefer `.toHaveLength()` over `.length.toBe()`:
+
+```typescript
+// Good
+expect(store.items).toHaveLength(1)
+
+// Avoid
+expect(store.items.length).toBe(1)
+```
+
+Use `.toMatchObject()` for partial matching:
+
+```typescript
+expect(store.completedItems[0]).toMatchObject({
+  id: 'task-123',
+  status: 'done'
+})
+```