Register UI options with custom CLI objects

[floitsch]

Summary

• Register UI arguments when a caller supplies its own Cli object and enables --add-ui-help.
• Clarify that a supplied Ui itself is unchanged; the flag controls accepted UI arguments and help.
• Add a regression test using CommandGroup, a custom base Cli, and --output-format=json.

Testing

• make test — 16/16 passed

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