[ENH] Fixes #1051: add hyperactive tuner as importable

[Atharv1405]

Reference Issues/PRs

Fixes #1051

What does this implement/fix? Explain your changes.

This PR implements the dynamic _placeholder_record forwarding pattern in skpro registry (similar to sktime Prophetverse integration) and registers the ProbaRegOptCV hyperactive-tuner wrapper as an importable placeholder class.

Specifically:

1. Added _placeholder_record decorator: Created skpro/registry/_placeholder_rec.py containing a dynamic registry utility decorator. If the soft dependency hyperactive is installed in the runtime environment, the class is transparently swapped with the actual external package class (hyperactive.integrations.skpro.ProbaRegOptCV). Otherwise, it keeps the stub class and wraps its init to raise a clean and descriptive soft-dependency ImportError on instantiation.
2. Created stub class ProbaRegOptCV: Created skpro/model_selection/_hyperactive.py to define the ProbaRegOptCV stub. This contains necessary search tags like "python_dependencies": "hyperactive" to enable registry indexing and documentation generators to find it without requiring the installation of hyperactive beforehand.
3. Namespace Registration: Exposed _placeholder_record in skpro/registry/init.py and exposed the tuner ProbaRegOptCV in skpro/model_selection/init.py.

Does your contribution introduce a new dependency? If yes, which one?

Yes, it introduces hyperactive as an optional soft dependency. It does not introduce any new hard dependencies

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

No direct standalone test was added

Any other comments?

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[fkiraly]

Thanks! This should have the tests:vm tag though so it runs on a VM.

[Atharv1405]

Added vm tag . i hope its correct now @fkiraly