fix: guard broadcast_and_apply from mixed backends

[agoose77]

This PR ensures that broadcast_and_apply throws an error if the given arrays do not share the same backend.

At a high level, all ak.XXX operations that accept multiple array arguments should effectively compute

backend = backend_of(*args, default=..., coerce_to_common=True)
layouts = [ak.to_layout(x).to_backend(backend) for x in args]

We could add a helper to do this, although given that each ak.XXX function needs to set different layout conversion flags, this is probably best avoided.

There's an argument to be made for making broadcast_and_apply be more permissive and perform the coercion itself. However, I think we want to coerce to the same backend in the highlevel ak.XXX functions ASAP, i.e. before broadcasting.

Whilst I was here, I cleaned up some of the routines that use backend_of.

[codecov]

Codecov Report

Merging #2678 (e1ea6cc) into main (dea0139) will increase coverage by 0.01%.
The diff coverage is 94.53%.

Additional details and impacted files

Files Changed	Coverage Δ
src/awkward/operations/ak_to_backend.py	100.00% <ø> (ø)
src/awkward/_broadcasting.py	95.43% <50.00%> (-0.21%)	⬇️
src/awkward/_backends/dispatch.py	92.85% <88.23%> (-1.18%)	⬇️
src/awkward/operations/str/akstr_join.py	93.33% <90.90%> (+0.65%)	⬆️
src/awkward/operations/str/akstr_repeat.py	95.12% <92.85%> (+0.67%)	⬆️
src/awkward/operations/ak_where.py	92.59% <93.75%> (-0.14%)	⬇️
src/awkward/_connect/numpy.py	91.89% <100.00%> (+0.07%)	⬆️
src/awkward/contents/content.py	76.16% <100.00%> (ø)
src/awkward/operations/ak_almost_equal.py	93.18% <100.00%> (+0.49%)	⬆️
src/awkward/operations/ak_argcartesian.py	88.57% <100.00%> (ø)
... and 20 more

[jpivarski]

This all looks good. Thanks!