NGAP: enable Handover Cancel encoding

This commit adds support for the Handover Cancel initiating message
procedure code (NGAP_ProcedureCode_id_HandoverCancel).

Changes:
- ngap_gNB_decoder.c: Added case handler for Handover Cancel in
  ngap_gNB_decode_initiating_message() to properly decode and log
  incoming Handover Cancel messages from the AMF.

- ngap_gNB_encoder.c: Added Handover Cancel to the list of supported
  initiating message procedure codes in ngap_gNB_encode_initiating(),
  allowing the gNB to encode and send Handover Cancel messages.

The Handover Cancel procedure is already implemented in other parts of
the NGAP stack, these changes were missing to complete the support
by ensuring the procedure code is properly recognized in both the encoder
and decoder switch statements, following the same pattern as other
procedure codes.

openair3/NGAP/ngap_gNB_decoder.c

@@ -76,6 +76,12 @@ static int ngap_gNB_decode_initiating_message(NGAP_NGAP_PDU_t *pdu) {
       NGAP_INFO("Handover Preparation initiating message\n");
       break;
 
+    case NGAP_ProcedureCode_id_HandoverCancel:
+      res = asn_encode_to_new_buffer(NULL, ATS_CANONICAL_XER, &asn_DEF_NGAP_NGAP_PDU, pdu);
+      free(res.buffer);
+      NGAP_INFO("Handover Cancel initiating message\n");
+      break;
+
     case NGAP_ProcedureCode_id_PDUSessionResourceModify:
       res = asn_encode_to_new_buffer(NULL, ATS_CANONICAL_XER, &asn_DEF_NGAP_NGAP_PDU, pdu);
       free(res.buffer);

openair3/NGAP/ngap_gNB_encoder.c

@@ -55,6 +55,7 @@ static inline int ngap_gNB_encode_initiating(NGAP_NGAP_PDU_t *pdu, uint8_t **buf
                                       NGAP_ProcedureCode_id_PDUSessionResourceModifyIndication,
                                       NGAP_ProcedureCode_id_UplinkRANStatusTransfer,
                                       NGAP_ProcedureCode_id_HandoverPreparation,
+                                      NGAP_ProcedureCode_id_HandoverCancel,
                                       NGAP_ProcedureCode_id_HandoverNotification};
   int i;
   for (i = 0; i < sizeofArray(tmp); i++)