[CLI] If an extension ties into an existing command group, and has only in_preview subcommands, the parent command group is being marked as in_preview

## Summary
When working on the health-models extension I noticed this:
- the extension is overwriting to get into `monitor health-models`
- all sub commands, e.g. `monitor health-models entity ....` are being marked as in_preview
- there seems to be a roll-up or mechanism in the extension code generator, that then will mark the whole `monitor ...` as in_preview

It might be an edge case, since the issue is only relevant, if there's a separate extension targeting an existing command group.

## Steps to reproduce
tbd

## Actual behavior
tbd

## Expected behavior
- The generator should verify if there might be others adding commands to the group that are not in_preview