BUG: Fix DatetimeIndex timezone preservation when joining indexes with same timezone but different units
- [x] closes #60080
- [x] [Tests added and passed]
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations]
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Hi @rhshadrach, could you please check this one out? I agree with @rhshadrach's comment on splitting/parametrizing the test, otherwise this LGTM Hi @rhshadrach, I see that you've requested changes. Could you please clarify what exactly needs to be changed? as I've already made the changes you've requested about `assert result.tz == idx1.tz` > Hi @rhshadrach, I see that you've requested changes. Could you please clarify what exactly needs to be changed? as I've already made the changes you've requested about `assert result.tz == idx1.tz`

Once changes are requested, the state doesn't update until another review is submitted. Thanks @myenugula 