test(ws): add unit tests for WebSocket ConnectionManager and heartbeat

[mijinummi]

🧪 Unit Tests: WebSocket Broadcaster & Heartbeat (#723)

📝 Summary

Adds comprehensive unit tests for the WebSocket ConnectionManager and heartbeat session lifecycle in stellarflow-backend/tests/test_websocket_manager.py.

closes #723

🛠️ Test Coverage

• Lifecycle: Verifies client connection state tracking and clean channel cleanup on disconnect.
• Pub/Sub Subscriptions: Tests topic subscription/unsubscription mapping logic (subscribe:pool:XLM-USDC).
• Broadcasting: Confirms JSON payload delivery across subscribed client WebSockets.
• Heartbeat Pruning: Confirms stale/unresponsive client connections are disconnected when heartbeat pings fail.

pytest stellarflow-backend/tests/test_websocket_manager.py

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