Catch shape issues for histogram filling before compute time

[alexander-held]

The following example fails at dask.compute time due to the array being used to fill the histogram not being flattened:

import uproot
import hist.dask


evts = uproot.dask({"https://github.com/scikit-hep/coffea/raw/refs/heads/master/tests/samples/nano_dy.root": "Events"})
res = hist.dask.Hist.new.Regular(10, 0, 10).Double().fill(evts["Jet_pt"])
res.compute()

After conversation with @lgray I believe this could (ideally should?) already be caught earlier to avoid a runtime error. The ultimate exception that is raised currently is:

ValueError: cannot convert to RegularArray because subarray lengths are not regular (in compiled code: https://github.com/scikit-hep/awkward/blob/awkward-cpp-44/awkward-cpp/src/cpu-kernels/awkward_ListOffsetArray_toRegularArray.cpp#L22)

environment:

Package            Version
------------------ -----------
awkward            2.7.4
awkward_cpp        44
boost-histogram    1.5.1
dask               2025.1.0
dask-awkward       2025.2.0
dask-histogram     2025.2.0
hist               2.8.0
uproot             5.5.1