Compatibility with Dask 2025.4.1

[TomAugspurger]

This PR is another attempt at fixing compatibility with dask upstreaming, targeting dask==2025.4.1

rapidsai/dask-upstream-testing#44 and linked CI runs as the list of failures. This PR fixes the first two classes of failures, one set in LabelBinarizer and one set in kneighbors. See the commit messages for links to failures.

There's one remaining class of failures in cuml/python/cuml/cuml/tests/dask/test_dask_random_forest.py::test_rf_broadcast[True-True-classification]. I'm currently planning to fix those in a separate PR since it looks more complicated. These should be fixed by 1d75861 and 22393bb.

[TomAugspurger]

CI failure at https://github.com/rapidsai/cuml/actions/runs/14882476121/job/41794658828?pr=6614#step:9:3455

_______________ test_qn[True-0.1-0.1-elasticnet-float64-softmax] _______________
[gw6] linux -- Python 3.10.17 /opt/conda/envs/test/bin/python
Traceback (most recent call last):
  File "/opt/conda/envs/test/lib/python3.10/site-packages/_pytest/runner.py", line 341, in from_call
    result: Optional[TResult] = func()
  File "/opt/conda/envs/test/lib/python3.10/site-packages/_pytest/runner.py", line 262, in <lambda>
    lambda: ihook(item=item, **kwds), when=when, reraise=reraise
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_hooks.py", line 513, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_manager.py", line 120, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_callers.py", line 182, in _multicall
    return outcome.get_result()
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_result.py", line 100, in get_result
    raise exc.with_traceback(exc.__traceback__)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_callers.py", line 103, in _multicall
    res = hook_impl.function(*args)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/_pytest/runner.py", line 177, in pytest_runtest_call
    raise e
  File "/opt/conda/envs/test/lib/python3.10/site-packages/_pytest/runner.py", line 169, in pytest_runtest_call
    item.runtest()
  File "/opt/conda/envs/test/lib/python3.10/site-packages/_pytest/python.py", line 1792, in runtest
    self.ihook.pytest_pyfunc_call(pyfuncitem=self)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_hooks.py", line 513, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_manager.py", line 120, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_callers.py", line 139, in _multicall
    raise exception.with_traceback(exception.__traceback__)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/pluggy/_callers.py", line 103, in _multicall
    res = hook_impl.function(*args)
  File "/opt/conda/envs/test/lib/python3.10/site-packages/_pytest/python.py", line 194, in pytest_pyfunc_call
    result = testfunction(**testargs)
  File "/__w/cuml/cuml/python/cuml/cuml/tests/test_qn.py", line 71, in test_qn
    assert cuml_score >= 0.50
AssertionError: assert 0.456 >= 0.5

That appears to be unrelated but I'm not sure.

[betatim]

I've restarted the failed test. I think the failure of the QN test is a sign of a flaky test, I've had it fail a few times (and then not) in another PR this week

[jcrist]

[jcrist]

/merge