BUG: ensure_datetime64ns with bigendian array
- [x] closes #29684
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think a test that directly hits the user-facing behavior of `pd.Series(big_endian_array)` would be valuable to ensure we don't regress in a future refactor. What do people think about backporting this? IMO it's appropriate for 1.0.1, but we don't have backports to that setup yet. So 1.0.0 is probably best.

We also need a whatsnew for this, but that depends on the decision for where this should go. I guess I view 1.0.1 as reserved for regressions introduced in 1.0 which this wouldn't fall under, so very soft preference for 1.1 but not losing sleep in either case -1 on backporting this; we have very little support for big-endian at all and no testing. In fact, @jbrockmendel how do you know this actually works? > In fact, @jbrockmendel how do you know this actually works?

Because you can pass a big-endian dtype even on a little-endian system.  The added test does this. > > In fact, @jbrockmendel how do you know this actually works?
> 
> Because you can pass a big-endian dtype even on a little-endian system. The added test does this.

disagree; we cannot reproduce the error. It would be ok to have the OP try to repro with this patch though. Not objecting to the change, but we don't have *any* testing whatsoever. > disagree; we cannot reproduce the error

We can.  Try the tests this adds on master. > > disagree; we cannot reproduce the error
> 
> We can. Try the tests this adds on master.

we don't have tests on a big-endian CI. > we don't have tests on a big-endian CI.

I'm aware.  I'm saying that when I run the test on my little-endian computer, it fails in master and passes after this PR. k thanks @jbrockmendel 

not harassing you about testing, just skeptical of big-endian testing generally