Bug Description: When the SMF sends a PFCP Session Establishment Request, a malicious/rogue UPF can reply with a PFCP Session Establishment Response that omits the mandatory NodeID IE. This causes SMF to call (pfcpType.NodeID).ResolveNodeIdToIp() on a nil NodeID pointer, leading to a runtime panic and process termination, resulting in a remote Denial of Service against the SMF. Reproduction Steps: 1. Start a fake UPF server. 2. Attach a UE and establish a PDU session using UERANSIM. 3. The rogue UPF responds with a PFCP Session Establishment Response that lacks the NodeID IE. Expected Behavior: SMF should validate mandatory IEs in Session Establishment Response and, if missing, reject the response gracefully without terminating the process. Environment**: - free5GC Version: v4.1.0 - OS: Ubuntu 22.04 Server - Kernel version: 5.15.0-0-generic - Go version: go1.24.9 linux/amd64