Update backend pool

Updates one backend pool of the load balancer ("Save pool" in the Backend pools tab): name, algorithm (ROUND_ROBIN | LEAST_CONNECTIONS | SOURCE_IP) and sessionPersistence (SOURCE_IP | HTTP_COOKIE | APP_COOKIE | null for none). Every one of these lives in the provider (Octavia), so the update is asynchronous: the request is accepted with 202, a tracking operation loadbalancer.load_balancer.pool.update is opened, and a worker applies the fields to the Octavia pool before updating the platform record. protocol cannot change on an existing pool. Validation: empty body → 422; parent LB and pool must exist in the scope (404); the LB must accept child changes — not PENDING_CREATE/PENDING_DELETE, not ERROR, and already present in the provider (409 CONFLICT); a protocol change or an invalid enum value → 422 VALIDATION_FAILED. Async & state: Always asynchronous: 202 with the pool as it will read once applied. Poll the operation (or watch resource.updated SSE frames, kind "network.load_balancer", patch to pools). Errors: 401 unauthenticated; 403 insufficient role; 404 unknown LB or pool; 409 state conflict; 422 invalid fields; 500 backend failure. Idempotency/Concurrency: Idempotency-Key is optional; a retry with the same key and body replays the original 202, a different body under the same key is 409. If-Match is not evaluated for pool updates.