When an ACP agent answered session/set_config_option with an error,
choose() only logged it, so the session's current value never moved and
offered_pickers() kept treating the refused model as pending: the picker
showed the refused model as current and hid the effort picker.
hive-runtime now keeps the refused value per category on Choices, exposed
as Choice::refused, set on the RPC error and cleared when the agent next
accepts a value for that category. offered_pickers() treats a refused
value as not settable, so the picker shows the session's actual model and
its effort levels.
The fake ACP agent gains REFUSE, which errors the first
session/set_config_option to that value.
Refs #4832 (ACP half only)
25a09dd3ec