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