BUG: Preserve Timezone and Higher Resolution in DatetimeIndex.union
- [X] closes #60080 (Ensure timezone and resolution are preserved in `DatetimeIndex.union` operations with the same timezone but different units)
- [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 `test_union_preserves_timezone_and_resolution`.
  - Added `test_union_different_timezones`.
- [X] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit):
  - Used `pre-commit` hooks for `flake8`, `isort`, and `black`.
- [X] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions:
  - Annotated the resolution comparison logic and ensured clarity for mappings.
- [X] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature:
  - Section: **Bug Fixes**
    - Entry: Fixed timezone and resolution preservation in `DatetimeIndex.union`. Previously, `DatetimeIndex.union` would sometimes convert timezone-aware indices with the same timezone but different units to UTC. GH#60080



This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. 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.