API: date_range, timedelta_range infer unit from start/end/freq

[jbrockmendel]

• closes ENH/BUG: pd.date_range() still defaults to nanosecond resolution #59031 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

Waiting to update expected's until after #62801, so this won't pass yet.

[jorisvandenbossche]

Should we only do this for pd.date_range for now, and not yet for pd.timedelta_range, since in general timedelta still defaults to nanoseconds? (eg when converting strings or integers in pd.to_timedelta)