When a user toggles off the last visible model for a provider group, the
effectiveVisibleKeys() function treated the missing provider prefix as
'never customized' and re-added the default models on the next render,
causing all models to snap back to enabled.
Fix: store a sentinel key (e.g. 'provider::') when the last model for a
provider is toggled off. The sentinel distinguishes 'user hid everything'
from 'user never customized', preventing the default-fallback path from
re-adding models the user explicitly chose to hide.
Fixes#43485
liuhao1024
93a2f680fd