List the external networks the scope may allocate floating IPs from (or attach to)

Projects the scope's external-network allow-list (GLOBAL classifications plus this project's assignments) for one region. With purpose=fip (default) it lists the networks the scope may allocate floating IPs from (allowFip=true) — the acquire-floating-IP form's Pool choices, the same list the create-floating-IP guard enforces. With purpose=attach it lists the networks the scope may put a VM interface directly on (allowAttach=true) — the create wizard's "Public interface" choices, the same list the instance-create attach gate enforces; those rows carry allowAttach. id is the Neutron network id. Validation: unknown or foreign scope → 404 NOT_FOUND; purpose outside fip|attach → 422 VALIDATION_FAILED. A region with no classified network is an empty list. Async & state: Synchronous read; no state change. region defaults to ist-1.