[nexus] add 1_3_DIAG_TC_1 for Network Diagnostic and Child Info (openthread#12789)

This commit adds the Nexus test case 1_3_DIAG_TC_1 which verifies
that a Thread Router correctly reports its child and neighbor
information via Network Diagnostic and MeshDiag queries, as per
the Thread 1.4 test specification.

The implementation includes:
- tests/nexus/test_1_3_DIAG_TC_1.cpp: Sets up a star topology with
  a Leader, Router_1 (DUT), and various child nodes (FED, MED, SED,
  REED). It triggers Network Diagnostic Get and MeshDiag queries
  (QueryChildTable, QueryChildrenIp6Addrs, QueryRouterNeighborTable)
  from the Leader to the DUT.
- tests/nexus/verify_1_3_DIAG_TC_1.py: Performs automated verification
  of the captured traffic. It implements a custom TLV parser for
  CoAP payloads to verify Max Child Timeout (19), Vendor/Stack info
  (23-28), MLE Counters (34), Child Table (29), Child IPv6 (30),
  and Router Neighbor (31) TLVs.
- Integrated the new test into tests/nexus/CMakeLists.txt and
  tests/nexus/run_nexus_tests.sh.

The test ensures the correctness of Thread 1.4 Router Diagnostic
and Child Information reporting, facilitating remote monitoring
and management of the Thread network.

Author: jwhui

tests/nexus/CMakeLists.txt

@@ -279,6 +279,7 @@ ot_nexus_test(1_3_SRPC_TC_1 "cert;nexus")
 ot_nexus_test(1_3_SRPC_TC_4 "cert;nexus")
 ot_nexus_test(1_3_SRPC_TC_5 "cert;nexus")
 ot_nexus_test(1_3_SRPC_TC_7 "cert;nexus")
+ot_nexus_test(1_3_DIAG_TC_1 "cert;nexus")
 
 # Misc tests
 ot_nexus_test(border_admitter "core;nexus")

tests/nexus/run_nexus_tests.sh

@@ -214,6 +214,7 @@ DEFAULT_TESTS=(
     "1_3_SRPC_TC_4"
     "1_3_SRPC_TC_5"
     "1_3_SRPC_TC_7"
+    "1_3_DIAG_TC_1"
 )
 
 # Use provided arguments or the default test list