_model_flow_custom() saved model.provider and model.base_url to disk
via its own load_config/save_config cycle, but never updated the
setup wizard's in-memory config dict. The wizard's final
save_config(config) then overwrote the custom settings with the
stale default string model value.
Fix: after saving to disk, also mutate the caller's config dict so
the wizard's final save preserves model.provider='custom' and the
base_url. Both the model_name and no-model_name branches are
covered.
Added regression tests that simulate the full wizard flow including
the final save_config(config) call — the step that was previously
untested.
89d8127772