[ENH] add VotingProbaRegressor - heterogeneous ensemble compositor

[Ashish-Kumar-Dash]

Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

Adds VotingProbaRegressor, a heterogeneous ensemble that fits multiple
probabilistic regressors on the same data and returns a Mixture distribution
of their predictions.

Probabilistic generalization of sklearn's VotingRegressor — instead of
averaging point predictions, it combines full predictive distributions via
weighted mixture.

Key decisions for the basis of this PR:

• Inherits from BaseMetaEstimator, BaseProbaRegressor — follows the
  Pipeline pattern for heterogeneous estimator lists
  (named_object_parameters, property-based _estimators)
• Capability tags use AND logic across components
• Delegates all distribution math to Mixture

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

No.

What should a reviewer concentrate their feedback on?

• The AND logic for capability tag cloning (lines 80-84) — this is different
  from single-estimator clone_tags used elsewhere
• Whether _estimators as a property (mirroring Pipeline._steps) is the
  preferred pattern here

Did you add any tests for the change?

The estimator is auto-discovered by the existing test framework via
get_test_params (3 parameter sets). All pass:

• test_all_estimators — 67 passed
• test_all_regressors — 21 passed

Any other comments?

No.

PR checklist

For all contributions

• I've added myself to the list of contributors with any new badges I've
  earned :-)
• The PR title starts with either [ENH], [MNT], [DOC], or [BUG].

For new estimators

• I've added the estimator to the API reference - in
  docs/source/api_reference/regression.rst
• I've added one or more illustrative usage examples to the docstring, in
  a pydocstyle compliant Examples section.
• If the estimator relies on a soft dependency, I've set the
  python_dependencies tag and ensured dependency isolation. (N/A — no soft
  dependencies)

[Ashish-Kumar-Dash]

@fkiraly among the commits you'll also find a minor comma fix regarding .all-contributorsrc