refactor: drop behavior broadcasting

[agoose77]

The behavior for one array given to broadcasting functions no longer needs to be broadcast against the other array behaviors. Thus, let's remove that restriction.

This would break users who introduce behaviors on array X, and expect ak.broadcast_arrays to merge them.

[agoose77]

5a2a95a is preparation for another PR that made sense here.

[codecov]

Codecov Report

Merging #2761 (0c4ddd8) into main (43adb65) will decrease coverage by 0.01%.
The diff coverage is 87.50%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/_broadcasting.py	95.43% <100.00%> (ø)
src/awkward/_connect/numexpr.py	90.66% <100.00%> (ø)
src/awkward/_connect/numpy.py	91.77% <100.00%> (ø)
src/awkward/_errors.py	78.86% <100.00%> (ø)
src/awkward/_layout.py	84.46% <100.00%> (ø)
src/awkward/_nplikes/__init__.py	80.95% <100.00%> (ø)
src/awkward/_nplikes/dispatch.py	96.00% <100.00%> (ø)
src/awkward/_slicing.py	86.59% <100.00%> (ø)
src/awkward/contents/content.py	76.16% <100.00%> (ø)
src/awkward/index.py	89.34% <100.00%> (ø)
... and 15 more

... and 1 file with indirect coverage changes

[jpivarski]

Now that string-broadcasting is built-in, no longer comes from ak.behavior, the broadcast_and_apply function no longer needs to ask for it as an argument, and that decouples n-ary → n-ary functions so that they don't have to mix behaviors anymore.

You're right that this changes how ak.broadcast_arrays works, and is therefore a technically breaking change, but it is extremely niche. Behaviors are generally thought of as global settings (Coffea, Vector), and arrays can carry their own behavior just in case they're not. Encountering different behavior on the inputs to ak.broadcast_arrays would be that surprising, exceptional case. Depending on it would be very weird.

I think this can go in a patch release. And I'm happy with the changes as they are here; please go ahead and merge when you're ready!