Add read_selection()/selection_exists()/selection_error() to
tool_backend_helpers: one provider string per category ('nous' = managed
Nous Tool Gateway, vendor name = direct with the user's own credentials,
no key ever written = legacy credential autodetect). Legacy configs are
interpreted at read time only (use_gateway: true => nous); nothing is
migrated on disk, and the DEFAULT_CONFIG-seeded stt.provider: local is
treated as never-configured.
_resolve_managed_fal_gateway / _resolve_managed_fal_video_gateway now
switch on that string: 'nous' routes managed only (unentitled => error
naming the selection), a stored vendor routes direct only (missing
FAL_KEY => error naming FAL_KEY and the selection, no silent managed
reroute), and FAL_KEY presence no longer selects the route. Krea's
model-driven managed interception now requires no stored provider (or
the managed selection) instead of merely provider != krea, and the
image/video registries map the 'nous' selection to the FAL plugin.
2dea073a1c