## Summary
- update the self-serve business usage-based limit message to direct
users to their admin for additional credits
- add a focused unit test for the self_serve_business_usage_based plan
branch
Added also:
If you are at a rate limit but you still have credits, codex cli would
tell you to switch the model. We shouldnt do this if you have credits so
fixed this.
## Test
- launched the source-built CLI and verified the updated message is
shown for the self-serve business usage-based plan

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.
Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.