BUGFIX: itemsize wrong for date32[day][pyarrow] dtype #57948

[mraabhijit]

• 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 and booleans.

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