Core patches for nascTime FRER inter-session / DC transport diversity support

[MohamedSeliem]

Summary

Three targeted patches to Simu5G v1.5.0, required to support nascTime's
IEEE 802.1CB FRER inter-session and dual-connectivity transport diversity
work. All patches are additive/guarded, no existing behavior changes
unless the new parameters are explicitly set.

Commits

1. GtpUser: add forceTunnelPeer

Adds an optional forceTunnelPeer string parameter to GtpUser. When set,
bypasses TFT/Binder-based tunnel destination routing entirely and tunnels
all outgoing traffic to the named peer, used by a secondary UPF (e.g.
upf2) to converge its GTP tunnel on a specific gNB regardless of normal
per-UE routing decisions.

Guarded to UPF/PGW ownerType_ only, so the bypass can never be
accidentally triggered on a base-station-type GtpUser (where it would
silently break intra-cell local delivery).

2. MAC: guard against unsigned underflow in macSduRequest

Fixes a real bug: under multi-DRB QOS_PF scheduling contention, a CID
can be granted fewer bytes than MAC_HEADER (2 bytes). The existing
allocatedBytes - MAC_HEADER subtraction on an unsigned type then
underflows to ~4.29e9, which trips a downstream queue-size check and
crashes with a misleading "configured queueSize too low" error. Clamps
to 0 instead, which degrades gracefully to an empty RLC PDU for that CID
that TTI rather than aborting the simulation.

Reproducible with 3 UEs × 4-5 DRBs under QOS_PF contention. Will also be
filed as an upstream issue against Unipisa/Simu5G separately.

3. IHandoverPacketFilter: add eNB-specific handover gate

This is to correct error in handover gate conncection.