[nexus] add SRPC-TC-5 for DNS-SD service discovery (openthread#12787)

This commit adds the Nexus test case 1_3_SRPC_TC_5 which verifies
that a DNS-SD client can correctly discover multiple services
registered via SRP, as per the Thread 1.3 test specification.

The implementation includes:
- tests/nexus/test_1_3_SRPC_TC_5.cpp: Executes the test sequence
  by configuring a Border Router (BR_1), an End Device (ED_2)
  registering 5 services with various TXT records, and a DUT
  (TD_1). It instructs the DUT to browse for services, resolve
  them, and send UDP packets to each resolved service. It verifies
  the TXT record values and successful UDP transmissions.
- tests/nexus/verify_1_3_SRPC_TC_5.py: Performs automated
  verification of the captured traffic (PCAP). It validates the
  DNS query, the DNS response containing all 5 services, and the
  subsequent UDP packets sent to the resolved addresses and ports.
- Integrated the new test into tests/nexus/CMakeLists.txt and
  tests/nexus/run_nexus_tests.sh.

The test ensures the correctness of DNS-SD client discovery logic
and its ability to handle multiple service responses in a Thread
network.

Author: jwhui

tests/nexus/CMakeLists.txt

@@ -277,6 +277,7 @@ ot_nexus_test(1_3_SRP_TC_13 "cert;nexus")
 ot_nexus_test(1_3_SRP_TC_15 "cert;nexus")
 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")
 
 # Misc tests
 ot_nexus_test(border_admitter "core;nexus")

tests/nexus/run_nexus_tests.sh

@@ -212,6 +212,7 @@ DEFAULT_TESTS=(
     "1_3_SRP_TC_15"
     "1_3_SRPC_TC_1"
     "1_3_SRPC_TC_4"
+    "1_3_SRPC_TC_5"
 )
 
 # Use provided arguments or the default test list