renderers: keep the classes a widget template sets

[dyve]

list_to_class assigned the enclosing div's class attribute outright, so any class the widget's own template set was discarded. RadioSelectButtonGroup sets btn-group btn-group-toggle, which is what makes it a button group, so rendering it through bootstrap_field produced neither a button group nor plain radios. data-toggle="buttons" survived, because only the class was overwritten, leaving Bootstrap's toggle JavaScript wired to nothing.

The class is now added to what the widget already set. Django's RadioSelect and CheckboxSelectMultiple templates emit no class on that element, so this is a no-op for every widget shipped with Django: I diffed the rendered output of category1 through category4, at default and small size, and it is byte-identical. Only category5, the custom widget, changes. There is a test for each half of that.

I also dropped the follow-up soup.find("div", {"class": classes}). It re-found the element already in hand, and it only worked as long as the class was assigned rather than appended, so it would have quietly stopped matching after this change.

One thing this does not address: the enclosing div now reads btn-group btn-group-toggle radio radio-success. The radio radio-success part is noise on a button group, but it is what post_widget_render passes for every RadioSelect subclass, and suppressing it per widget is a wider change than this fix.

Fixes #894

🤖 Generated with Claude Code

[coveralls]

coverage: 87.091% (+0.06%) from 87.035% — fix/894-preserve-widget-classes into main