When every registered deep provider rejected a target, select() returned
an intentionally unbound success for any concrete environment selector,
including one that names no registered provider. A misconfigured
REA_ANALYSIS_PROVIDER therefore opened the target unbound, and the later
deep-operation diagnostic blamed target support instead of the unknown
configuration. The same identifier requested explicitly already failed
with unknown_provider.
Take the unbound shortcut only for a registered environment preference.
An unknown configured identifier now reaches concrete selection and fails
with unknown_provider, matching the explicit request. Registered
preferences and auto still leave artifact-only targets unbound.
Fixes#2592