BUG: Fix Series.str.zfill for ArrowDtype string arrays #61485
Implemented `_str_zfill` for `ArrowExtensionArray` to support `Series.str.zfill` on Arrow-backed string arrays (`ArrowDtype(pa.string())`). This fixes an AttributeError due to the method relying on `_str_map`, which wasn't implemented. Used `_apply_elementwise` to match the approach of other string methods. Added tests under `test_string_array.py` and confirmed they pass. Also confirmed no other relevant test files are broken and the change aligns with how other string accessors are handled.

- [x] closes #61485
- [x] Tests added and passed
- [x] All code checks passed with pre-commit
- [x] Added a changelog entry in v3.0.0.rst
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