Fix concat failure with pandas StringDtype indexes (#11317)

[SAY-5]

Description

Fixes #11317. When pandas returns a StringDtype-backed Index (e.g. with future.infer_string enabled or pandas 3.x defaults), passing it as the new concat dim caused _calc_concat_dim_index to store the StringDtype as the resulting PandasIndex.coord_dtype. A subsequent concat against another input with a numpy <U* coord then failed in np.result_type with TypeError: Cannot interpret '<StringDtype...>' as a data type. PandasIndex.__init__ already coerces non-allowed extension dtypes via get_valid_numpy_dtype when coord_dtype is None; this PR reuses that path for StringDtype so the resulting coord ends up as object and concat succeeds. Regression test added.

Checklist

• Closes concat fails due to StringDtype introduced by pd.Index #11317
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude. I directed the agent to localize the failure to _calc_concat_dim_index in xarray/structure/concat.py, evaluated alternative fix sites (PandasIndex.concat vs the dim-index helper), reviewed and tested the resulting patch, and confirmed the regression test fails on main and passes here.

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