Add comprehensive end-to-end tests for the WebSocket system:
- Protocol tests: auth (valid/invalid token, timeout, double auth),
subscriptions (valid/invalid event, auth required, unsubscribe),
message delivery (notification on team add, doer exclusion,
multi-connection)
- Frontend integration tests: notification badge update, dropdown
rendering, and logout cleanup via browser-level Playwright tests
- Comment notification test: full flow where user B mentions user A
in a task comment and user A receives real-time WebSocket notification
Includes ws test dependency, shared test helper utilities, and
cascade-truncation of notifications when truncating users to prevent
test pollution.
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