BUG: Handle unit count when converting from `np.datetime64`
- [x] closes #39977
- [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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature.

I think this case is rare enough I’d rather not support it at all than support is inconsistently across the codebase.  Iirc @seberg was thinking of deprecating it in numpy too Yeah, I suspect very few use that really and unless someone would shout, I would be fine with deprecating it.  I would also suspect NumPy also has bugs around it due to it having at least almost no users in practice. Any interest in adding a warning or putting a note in the docs? Otherwise, I'll close this PR. Looks like there not much appetite for this change so closing 