Merge remote-tracking branch 'origin/ue_nas_simulator' into integration_2026_w13 (!3973)

Introduce UE NAS Simulator

This MR introduces a simple standalone tester for 5G UE NAS signaling and NGAP
messages. It connects to the AMF to perform NGAP and NAS encoding/decoding and
validates the UE Registration and PDU Session Establishment processes without
the overhead of a full Radio Access Network (PHY/MAC/RLC).

The motivation for this tester is as follows,
- This tester is a lightweight, deterministic tool to test the UE NAS layer
directly.
- Developers can easily inject specific NAS messages depending on the use-case.
- Developers can validate custom NAS/NGAP messages without the need to implement
complete RRC/MAC etc procedures.

Start the 5G Core network to test the UE NAS simulator. The compiling and
testing procedure is described below.
To compile:
mkdir build && cd build && cmake .. -GNinja && ninja nr-ue-nas-simulator-test

To run:
  LD_LIBRARY_PATH=. ./tests/nr-ue-nas-simulator/nr-ue-nas-simulator-test -O ../tests/nr-ue-nas-simulator/test.conf

doc/README.md

@@ -124,6 +124,7 @@ The other SDRs (AW2S, LimeSDR, ...) have no READMEs.
 - Component tests are under `tests/`. Currently, the following component tests are supported
   - A simple CU-UP tester, see the corresponding [README.md](../tests/nr-cuup/README.md).
   - A CU-NRPPA tester, see the corresponding [README.md](../tests/nr-cu-nrppa/README.md).
+  - A UE NAS tester, see the corresponding [README.md](../tests/nr-ue-nas-simulator/README.md).
 - [TESTBenches.md](./TESTBenches.md) lists the CI setup and links to pipelines
 - The CI setup uses a [custom framework](../ci-scripts/README.md) to run
   end-to-end tests.