test: add tests for WebSocket store

[d3vobed]

Adds comprehensive tests for the WebSocket store:

• Initial state verification
• Connection actions: setStatus, setUrl, markConnected (resets reconnect attempts), markDisconnected, incrementReconnectAttempts, resetReconnectAttempts, setMaxReconnectAttempts
• Subscription actions: subscribe (pending), confirmSubscription (moves to active), unsubscribe (active and pending), clearPendingSubscriptions, isSubscribed
• Message actions: addMessage (auto-id, in reverse order), clearMessageHistory, setLastMessage(null)
• Error actions: addError (reverse order), clearErrors
• Reset: full state reset to initial values
• Selectors: selectWebSocketStatus, selectIsConnected, selectActiveChannels, selectLastMessage, selectMessagesByChannel, selectConnectionStats
• Edge cases: duplicate subscribe on active channel, unsubscribe non-subscribed channel, confirm non-pending subscription, empty message history, empty errors

Follows existing store test conventions from notificationStore.test.ts.

Closes #764

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