fix(playground): update error handling to show unhandled errors in playground (#5442)

* feat(playground): surface ability to set api keys as env vars on server in ui

* keep local message content and tool calls around for ease of switching back and forth

* use error message for subscription onError

* remove todos

* parse and display unhandled errors in playground output

* remove extraneous file

* add tests

* remove default max_tokens as it was ignored anyways