test(chat): Sprint 5 -- unit tests, E2E tests, feature parity validation
- Add hub_test.go: register/unregister, join/leave room, broadcast, exclude sender,
send to user, multiple clients same user (6 tests)
- Add handler_messages_test.go: send message, missing fields, edit ownership check,
soft delete (4 tests)
- Add handler_realtime_test.go: typing broadcast, read receipts, reactions add/remove,
delivered status (5 tests)
- Add e2e_chat_ws_test.go: auth valid, missing token, invalid token, ping/pong
- Add e2e_chat_messages_test.go: 2-client message flow, typing indicator
- Create CHAT_FEATURE_PARITY.md: 25-feature checklist (all OK or IMPROVED)
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