Align full-DNN construction with 3GPP TS 23.003 §9.1.2 by switching the
Operator Identifier format from "5gc.mncXXX.mccYYY.3gppnetwork.org" to
"mncXXX.mccYYY.gprs". Introduce new helper utilities to extract and build
OI (Operator Identifier) from both PLMN-ID and FQDN, and replace the
legacy `ogs_home_network_domain_from_fqdn()` usage in AMF/SMF/PCF paths.
This resolves DNN misalignment in vSMF–hSMF PDU Session Create that
caused interop issues with external 5G core vendors during HR roaming.
Includes updates across AMF/SMF/PCF, unit tests, and supporting helpers.
Issues: #4096
