Fixes a panic in the websocket proxyd logic. Normally, the `clientPump` and `backendPump` methods in `WSProxier` send data in one direction. However, when the client sends an invalid RPC, the `clientPump` will send a response _directly to the client_ in order to avoid unnecessary roundtrips to the backend. This could be interleaved with concurrent writes to the client's WS in `backendPump`, and would cause a panic in the WS library.

To test this, this PR includes a dedicated integration test that reliably triggers the issue. In addition, this PR adds additional testing for WS functionality.