[nexus] add test for IPv6 default route advertisement (PIC-TC-3) (#12860)

This commit adds a Nexus test case 1_4_PIC_TC_3 to verify the IPv6
default route advertisement behavior of a Border Router (BR) in a
Thread 1.4 network.

The test verifies that:
- The BR correctly advertises a default route (::/0) in Thread Network
  Data when it discovers a default route on the infrastructure link.
- The BR maintains the default route advertisement even if the
  infrastructure default route is withdrawn, provided a non-ULA prefix
  remains active on the infrastructure link.
- The default route advertisement is correctly restored or updated when
  the infrastructure default route is re-enabled.

The test implementation includes:
- test_1_4_PIC_TC_3.cpp: C++ test logic using the Nexus simulation
  framework, simulating BR, Router, End Device, and Infrastructure
  nodes (Eth_1, Eth_2). It uses a custom ICMPv6 receive callback to
  simulate "no route to host" conditions.
- verify_1_4_PIC_TC_3.py: Python verification script that analyzes
  the captured packets to ensure MLE Data Responses and ICMPv6 traffic
  match the expected behavior for each test step.

Integration:
- Updated tests/nexus/CMakeLists.txt and tests/nexus/run_nexus_tests.sh
  to include the new test in the automated test suite.

tests/scripts/thread-cert/pktverify/layer_fields.py

@@ -657,6 +657,7 @@ _LAYER_FIELDS = {
     'thread_nwd.tlv.border_router_16': _list(_auto),
     'thread_nwd.tlv.has_route.br_16': _list(_auto),
     'thread_nwd.tlv.has_route.pref': _list(_auto),
+    'thread_nwd.tlv.has_route.np': _list(_auto),
     'thread_nwd.tlv.sub_tlvs': _list(_str),
     # TODO: support thread_nwd.tlv.prefix.length and thread_nwd.tlv.prefix.domain_id
     'thread_nwd.tlv.prefix': _list(_ipv6_addr),