test(ui-common): create shared mock WebSocket factory

test(ui-common): create shared mock WebSocket factory

Create ui/common/tests/mocks.ts with createMockWebSocketLike() — a factory
returning a MockWebSocketLike with trigger methods (triggerOpen, triggerClose,
triggerError, triggerMessage) and sentMessages capture array. Replace the
65-line inline createMockWs() in WebSocketClient.test.ts with an import
from the shared factory.