BUG: Categorical.map() errors when mapping categories to tuples

[SharadhNaidu]

Closes #51488

Categorical.map() (and by extension Series.map() on a categorical
Series) raised a NotImplementedError when the mapper produced tuple
values for the categories. Mapping the categories through Index.map()
returns a MultiIndex for tuple results, and a CategoricalDtype/
Categorical can't be built from a MultiIndex.

Detect that case and fall back to a flat object-dtype Index of tuples
(via to_flat_index()), matching how Index.map() already behaves when
a mapper returns tuples - so a categorical Series and a regular Series
now behave consistently here.

• closes BUG: categorical Series.map() errors when mapping to tuples. #51488
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