Add backend pool

Adds a new backend pool to the load balancer from the inline "Add pool" row in the Backend pools tab: name, protocol (HTTP | HTTPS | TCP | PROXY) and balancing algorithm (ROUND_ROBIN | LEAST_CONNECTIONS | SOURCE_IP). The pool starts with no members, no health monitor, sessionPersistence=null and operating=NO_MONITOR; it becomes discoverable in the LB's pools array when accepted and can then receive targets and a monitor. Validation: parent load balancer must exist in the scope and not be PENDING_DELETE (404 / 409 CONFLICT); name non-empty and unique among the LB's pools (409 CONFLICT); protocol and algorithm must be one of the supported enum values (422 VALIDATION_FAILED); sessionPersistence, when supplied, must be SOURCE_IP, HTTP_COOKIE or APP_COOKIE (422); contributor/owner/admin role is required (403). Async & state: Asynchronous — returns 202 with the LbPool resource (operating=NO_MONITOR) while the parent LB passes through provisioning=PENDING_UPDATE → ACTIVE. resource.updated SSE frames (kind "network.load_balancer", patch to pools and provisioning) refresh the pools table. Errors: 401 unauthenticated; 403 insufficient role; 404 unknown load balancer; 409 duplicate pool name or LB state conflict; 422 invalid protocol/algorithm/persistence; 500 backend failure. Idempotency/Concurrency: Idempotency-Key is required; replaying the same key returns the originally created pool instead of creating a duplicate.