Preserve decorated CLI command classes

[ktz03]

What does this PR do?

@optimum_cli_subcommand() registers a command but returns None, replacing the decorated class binding. The registered CLI command still runs, but the class cannot be instantiated, subclassed or reused in a CommandInfo for static registration.

Return the original command class after registration. The two regression cases cover root commands and children of the existing ExportCommand: the decorated class remains usable for static registration and execution, and registration through the actual CLI still works.

Local validation on Windows, Python 3.12:

• Real public decorator, constructor, subclass, CommandInfo and CLI probe: unchanged main has 7 failures and 5 normal controls; the fix passes all 12 cases.
• Relevant CLI tests: 4 passed, including both new regressions. Both new regressions fail on unchanged main.
• Repository-wide Black and Ruff checks passed with the official development versions.

The model-export CLI test was deselected. Full model/export integrations, accelerator backends, other operating systems and performance were not tested.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes? No documentation update is needed for this bug fix.
• Did you write any new necessary tests?