BUG: ensure to_numeric down-casts to uint64 for large unsigned integers
[to_numeric(..., downcast="unsigned")](cci:1://file:///d:/Github/pandas/pandas/core/tools/numeric.py:48:0-315:21) failed to honour the requested
[uint64](cci:1://file:///d:/Github/pandas/pandas/tests/tools/test_to_numeric.py:580:0-586:44) dtype when values exceeded `np.iinfo(np.int64).max`, returning
`float64` instead and losing integer precision (GH #14422 /
[test_downcast_uint64](cci:1://file:///d:/Github/pandas/pandas/tests/tools/test_to_numeric.py:580:0-586:44)).  
Added a fallback that detects integral, non-negative float results and
safely casts them to `np.uint64`.  All existing logic remains unchanged
for other code paths; the previously xfailed test now passes.

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.