Added 5 test cases covering:
1. Basic usage - subscribe/unsubscribe multiple topics
2. All unsubscribed - verify empty array after unsubscribing all
3. After close - verify returns null after websocket is closed
4. Duplicate subscriptions - verify topic only appears once
5. Multiple cycles - subscribe/unsubscribe/resubscribe patterns
All 5 tests pass with 24 expect() calls total.