Fix idxminmax for interval coordinates

[lavaFreak]

Description

Fix DataArray.idxmax() and DataArray.idxmin() when the reduced dimension uses interval coordinates. _calc_idxminmax() was replacing the coordinate's original pandas index adapter with a raw extension array before the final label lookup, which causes scalar interval selections to fail with TypeError: len() of unsized object.

This keeps pandas extension-backed coordinates on their original adapter for the final lookup, adds a regression test for both idxmax and idxmin, and documents the bug fix in doc/whats-new.rst.

Checklist

• Closes .idxmax() fails if coordinates are intervals #11300
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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