Improper Input Validation

Summary

free5GC's NEF terminates the entire process when a stored PFD-subscription `notifyUri` cannot be reached. In `PfdChangeNotifier.FlushNotifications()`, the notifier calls `NnefPFDmanagementNotify(...)` and on any delivery error invokes `logger.PFDManageLog.Fatal(err)`, which is `os.Exit(1)`-equivalent in Go. An attacker who can create a PFD subscription with an attacker-chosen `notifyUri` and then trigger a PFD change can deterministically kill NEF on the asynchronous delivery attempt -- the process exits with status `1`, dropping NEF's entire SBI surface until restart. This is materially worse than a per-request panic-DoS (Gin recovery does not catch `Fatal`). This issue affects versions prior to 1.2.3.