[Task]: Smoother mux handling of workspaces with no model set

[danbarr]

Description

If a client is configured to use the mux endpoint, but the active workspace has no model set, CodeGate returns a "404 status code (no body)" error through the client.

Two approaches I thought about:

1. Have CodeGate return a clear message about the workspace needing a model set (similar to how we handle codegate commands?)
2. Allow the user to (optionally) set a default model to be applied to all mux requests that come through workspaces with no model explicitly selected.

Additional Context

No response