[BUG] Return a boolean fitted state from MultiNormalizer

[marco-trotta1]

LLM generated content, by GPT-5.6

Reference Issues/PRs

Fixes #1806.

What does this implement/fix? Explain your changes.

MultiNormalizer now reduces delegated __sklearn_is_fitted__ results with all(). Other delegated methods continue to return their result lists.

What should a reviewer concentrate their feedback on?

• Confirm that partially fitted pipeline normalizers report the whole MultiNormalizer as unfitted.
• Confirm that result lists remain unchanged for methods other than __sklearn_is_fitted__.

Did you add any tests for the change?

Yes. The regression test covers unfitted, partially fitted, and fully fitted pipeline normalizers.

Any other comments?

The focused tests pass on macOS with Python 3.11. The repository-wide legacy model integration tests encounter existing MPS-specific failures.

PR checklist

• The PR title starts with [BUG].
• Added or modified tests.
• Ran all pre-commit hooks.

Scoped Ruff 0.6.9 format and lint checks passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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Flag	Coverage Δ
cpu	88.29% <100.00%> (?)
pytest	88.29% <100.00%> (?)

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