Reopening the Access Settings dialog after a save could leave the
radio on the wrong policy: the form initialized `access_policy`
from the route list's snapshot, but the list isn't refreshed when
the dialog closes, so the snapshot's value silently shadowed the
just-saved policy.
Backend now returns the route's authoritative `access_policy` on
`GET /v2/model-routes/{id}/access`. Use that value when the GET
resolves, falling back to the parent snapshot only as the initial
seed so the radio isn't briefly unselected.
While we're here, widen the `access_policy` type from a closed
union to `string` — plugins can contribute additional policy
values via the `accessControl.allowedUsersOverride` slot, so the
wire type shouldn't restrict to the OSS-side enum.
