[nexus] add test 1_4_PIC_TC_4 for NAT64 connectivity (#12862)

This commit implements the Nexus test specification 1_4_PIC_TC_4
to verify the Border Router (BR) built-in NAT64 translator.

The test verifies that the BR DUT:
- Automatically configures an IPv4 address and NAT64 prefix.
- Offers IPv4 internet connectivity to Thread devices using NAT64.
- Offers IPv4 local network connectivity to Thread devices.
- Operates a DNS recursive resolver to look up IPv4 server addresses.

New files added:
- tests/nexus/test_1_4_PIC_TC_4.cpp: C++ test execution script
- tests/nexus/verify_1_4_PIC_TC_4.py: Python pcap verification script

Integration:
- Updated tests/nexus/CMakeLists.txt to compile the test.
- Added test to default array in tests/nexus/run_nexus_tests.sh.

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

@@ -422,6 +422,7 @@ DNS_TYPE_PTR = 12
 DNS_TYPE_TXT = 16
 DNS_TYPE_AAAA = 28
 DNS_TYPE_SRV = 33
+DNS_TYPE_ANY = 255
 DNS_RCODE_NOERROR = 0
 DNS_RCODE_NXDOMAIN = 3