Wake task polling on terminal notifications

[HarperZ9]

Summary

• wakes the active task polling loop when a matching notifications/tasks/status terminal update arrives
• caches the latest notified task status so a terminal notification received just before the polling wait is not missed
• adds regression coverage proving a task with a long poll interval fetches its result immediately after a terminal notification

Closes #1037

Verification

• Red check before implementation: npm.cmd test -- --runTestsByPath src/__tests__/App.taskPolling.test.tsx --runInBand failed because the result remained Task created: task-notified-789. Polling for status...
• Green check after implementation: npm.cmd test -- --runTestsByPath src/__tests__/App.taskPolling.test.tsx --runInBand -> 3 passed
• npx.cmd prettier --check client/src/App.tsx client/src/__tests__/App.taskPolling.test.tsx -> passed
• git diff --check client/src/App.tsx client/src/__tests__/App.taskPolling.test.tsx -> passed
• npm.cmd run build-client -> passed