Ella Core has handover failures during concurrent Security Mode Command

Summary

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).

Impact

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.

Fix

Ella Core now enforces both rules from §6.9.5.1, blocking concurrent Security Mode Command and N2 handover procedures.