[nexus] add test 5.2.3 Leader rejects Address Solicit (2-hops away) (#12392)

Adds a new Nexus test case for 'Leader rejects CoAP Address Solicit
(2-hops from Leader)' (5.2.3) as specified in the test specification.

Summary of changes:
- Implemented Nexus test 5.2.3:
    - Added test_5_2_3.cpp: Sets up a topology with a Leader, 31 routers
      (fully connected to Leader), and a 32nd router (Router 32) that is
      2-hops away from the Leader via Router 1. Verifies that the Leader
      rejects the Address Solicit Request from the 33rd router with a
      'No Address Available' status (1).
    - Added verify_5_2_3.py: PCAP verification script for test 5.2.3.
      Ensures the Address Solicit Request is sent by Router 32 to the
      Leader, and that the Leader responds with a CoAP ACK containing
      a Status TLV with value 1 (NL_NO_ADDRESS_AVAILABLE).
- Updated build and execution scripts:
    - Modified CMakeLists.txt to build the new 5.2.3 test executable.
    - Updated run_nexus_tests.sh to include 5.2.3 in the default test
      list.

tests/nexus/platform/nexus_node.cpp

@@ -103,6 +103,8 @@ void Node::AllowList(Node &aNode)
 
 void Node::UnallowList(Node &aNode) { Get<Mac::Filter>().RemoveAddress(aNode.Get<Mac::Mac>().GetExtAddress()); }
 
+void Node::SetName(const char *aPrefix, uint16_t aIndex) { mName.Clear().Append("%s %u", aPrefix, aIndex); }
+
 #if OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE
 void Node::GetTrelSockAddr(Ip6::SockAddr &aSockAddr) const
 {