BUG: Fix ArrowDtype.itemsize for fixed-width types

[george-adams1]

• closes BUG: itemsize wrong for date32[day][pyarrow] dtype? #57948
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Fixes #57948

Summary

ArrowDtype.itemsize was incorrectly returning 8 bytes for date32[day] and other fixed-width PyArrow types because it always fell back to numpy_dtype.itemsize. This PR uses PyArrow's bit_width for fixed-width types and gracefully falls back to numpy for variable-width types.

Changes

• Modified ArrowDtype.itemsize to use pyarrow_dtype.bit_width when available
• Added comprehensive regression test covering the fix

Example

Before: ArrowDtype(pa.date32()).itemsize == 8
After: ArrowDtype(pa.date32()).itemsize == 4

[jbrockmendel]

Can you get the CI passing

[george-adams1]

Yup. I will work on this tomorrow.

[mroeschke]

Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.