Adds electron/gateway-ws-probe.cjs: a small helper that opens a gateway
WebSocket URL and classifies the handshake (open/frame → ok; error or close
before open → fail; open-then-early-close → credential rejected; never-opens →
timeout). The WebSocket implementation is injected so it can be unit-tested
without a real socket.
Wires gateway-ws-probe.test.cjs into test:desktop:platforms, covering every
handshake outcome plus constructor-throw and missing-impl.
10c78bf625