Independent review (APPROVED clean) + greptile both flagged one non-blocking nit:
_is_known_model_provider swallowed a transient _is_plugin_model_provider failure
at debug level, so a real plugin-backed provider could briefly vanish from the
picker with no visible log. Promoted to logger.warning(exc_info=True).
Also corrected the endpoint-error-hint comments in ui.js: they cited #4253, which
is actually a cron release (mislabel carried from #4279's body). Comment-only —
no logic change to the browser-verified collapsible-groups code.