Consider removing `commons_server()` from API

And using shinychat::chat_server() instead.

Currently, commons_server() does 3 things:

1. Creates a commons_server_start span.
   • Could be removed considering that Shiny already has instrumentation for the session's lifecycle?
2. Creates a conversation id.
   • This is a hack (that I added) and can go away when we have https://github.com/posit-dev/shinychat/pull/343
3. "Pre-warms" the client (i.e., builds the index, etc).
   • We might want to revisit how this works, but regardless, it might be better if this was explicit rather than implicit. Essentially the (recommended) API would change from:

client <- ...
commons_server("chat", client)

to

client <- ...
client$prewarm()
chat_server("chat", client)

I'm not sure if it's possible today, but at least at some point it should be possible to prewarm/cache the index independently of the shiny runtime. In that case, you might not want the pre-warming at all?

Start by tracing the commons_server() entry point and its three responsibilities: the commons_server_start span, conversation ID creation, and client pre-warming. Compare these with shinychat::chat_server() and the proposed client$prewarm() flow. Done means the API direction and treatment of each responsibility are decided, including whether pre-warming remains implicit.