Ella Core: Handover failures during concurrent Security Mode Command

Ella Core is a 5G core designed for private networks. Prior to 1.10.0, Ella Core didn't enforce security rules on concurrent running of security procedures defined in TS 33.501 §6.9.5.1 — it could send a NAS Security Mode Command while an N2 handover was still pending (and vice versa). Concurrent Security Mode Command and N2 handover produce a KgNB mismatch between the UE and target gNB, causing the handover to fail. Requires a stalled gNB + re-registration race to trigger. This vulnerability is fixed in 1.10.0.