Replace `RuntimeError` with `WebSocketDisconnected` once the socket has been disconnected

[ruitcatarino]

Discussed in #2762

[proto-atlas]

I can take a look at this. From the discussion, it sounds like the intended path is to use the existing disconnect exception rather than add another state flag. I'd update the WebSocket.send() disconnected-state branch to raise WebSocketDisconnect() instead of RuntimeError, update test_duplicate_close to expect it, and leave the receive-after-disconnect branch unchanged since that is a separate path. Does that match what you had in mind?