Merge remote-tracking branch 'origin/n2-handover' into integration_2025_w38 (!3334)

N2 handover

This MR introduces full support for N2 handover, covering handover
decision, preparation, execution, and notification between source and
target NG-RAN nodes.

Key features:

1. Handover Decision & Preparation:
  - Implement N2 handover decision based on UE measurement reports.
  - Generate the Handover Preparation Information message, including
    RRCReconfiguration.
  - Send Handover Required message from the source NG-RAN to the AMF.
2. Handover Execution:
  - Decode and handle the Handover Request on the target NG-RAN.
  - Process the Handover Command on the source gNB, triggering
    RRCReconfiguration.
  - Initiate Handover Notify after successful completion.
3 NGAP & RRC Enhancements:
  - Implement NGAP encoding/decoding functions for Handover Required,
    Handover Request, Handover Command, Handover Failure, and Handover
    Notify (3GPP TS 38.413).
  - Adds NG-RAN Status Transfer support (UL/DL) for PDCP COUNT
    preservation during mobility:
    - support for NGAP Uplink and Downlink RAN Status Transfer messages
      (TS 38.413 §9.2.3.13-14) to enable proper PDCP COUNT value
      transfer during handover or recovery: NGAP encoding/decoding for
      RAN Status Transfer, PDCP helpers to extract and apply COUNT (HFN
      + SN) per DRB. This aligns with TS 38.300 §9.2.3.2.1 and TS 23.502
      §4.9.1.3.3, ensuring PDCP status preservation in AM.
4. Testing:
  - Introduce a Telnet command to manually trigger N2 handover using UE
    ID and neighbor PCI.
  - Add configuration file for testing N2 handover with a target gNB.

ci-scripts/yaml_files/5g_f1_rfsimulator/docker-compose.yaml

@@ -110,6 +110,7 @@ services:
             public_net:
                 ipv4_address: 192.168.71.150
         volumes:
+          - ../../conf_files/neighbour-config.conf:/opt/oai-gnb/etc/neighbour-config.conf
           - ../../conf_files/gnb-cu.sa.band78.106prb.conf:/opt/oai-gnb/etc/gnb.conf
         healthcheck:
             test: /bin/bash -c "pgrep nr-softmodem"