BUG: Preserve Timezone and Higher Resolution in DatetimeIndex.union

[brandonmonge]

• closes BUG: Timezone upon DatetimeIndex union is changed to UTC #60080 (Ensure timezone and resolution are preserved in DatetimeIndex.union operations with the same timezone but different units)
• Tests added and passed if fixing a bug or adding a new feature:
  • Added test_union_preserves_timezone_and_resolution.
  • Added test_union_different_timezones.
• All code checks passed:
  • Used pre-commit hooks for flake8, isort, and black.
• Added type annotations to new arguments/methods/functions:
  • Annotated the resolution comparison logic and ensured clarity for mappings.
• 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

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update and respond to this comment if you're still interested in working on this.

[mroeschke]

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.