k8s: improve robustness for WebSocket streaming

#16678 fixed up a specific issue, but left a couple of rarer edge cases unaddressed. It would be good to explore migrating that code to the github.com/coder/websocket library for improved robustness in WebSocket frame parsing. We already have a dependency on it in the repo, and we only need to depend on it from k8s-operator and k8s-proxy, so it shouldn't be too impactful as a dependency.