BUG: Preserve timezone in numpy_dtype for ArrowDtype
- [x] closes #60237
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
* Added tests in pandas/tests/dtypes/test_dtypes.py to ensure numpy_dtype preserves timezone information for ArrowDtype.
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
* Type annotations are already present in the modified numpy_dtype function.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.
* Added a note in the bug fixes section: `- Fixed an issue where .convert_dtypes(dtype_backend="pyarrow") would strip timezone information from timezone-aware PyArrow timestamps. [GH#60237](https://github.com/pandas-dev/pandas/issues/60237)
`


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