[nexus] add DBR-TC-03 test for multi-Thread network reachability (#12735)

This commit introduces the 1_3_DBR_TC_3 nexus test case to verify
bi-directional reachability between multiple Thread networks connected
via a common infrastructure link.

The test verifies that independent Thread networks, each with its own
Border Router (BR) connected to the same infrastructure link, can
successfully route traffic to each other. This confirms reachability
in multi-Thread network environments where no existing IPv6
infrastructure is present.

Key changes:
- Implement test_1_3_DBR_TC_3.cpp to simulate a topology with two
  Thread networks (BR_1/ED_1 and BR_2/ED_2) and verify end-to-end
  ping success between End Devices.
- Implement verify_1_3_DBR_TC_3.py for pcap-based validation of:
    - Network Data registration of OMR and infrastructure prefixes.
    - RA multicasts on the infrastructure link with correct RIO/PIO.
    - Proper mapping of Extended PAN ID into the infrastructure ULA.
    - Bi-directional ICMPv6 connectivity between End Devices.
- Register the new test case in CMakeLists.txt and run_nexus_tests.sh.