[nexus] implement test 1_4_DNS_TC_3 for upstream DNS resolver selection (#12835)

This commit implements the Nexus test specification 1_4_DNS_TC_3 for
upstream DNS resolver selection in OpenThread.

Nexus Platform Enhancements:
- Added OPENTHREAD_CONFIG_DNS_UPSTREAM_QUERY_ENABLE and
  OPENTHREAD_CONFIG_PLATFORM_DNS_ENABLE to nexus config.
- Implemented platform DNS APIs in nexus_dns.cpp, supporting
  upstream server selection based on prefix lifetimes and reachability.
- Added UdpHook to Core to allow tests to intercept and simulate
  responses for backbone UDP traffic on port 53.
- Updated InfraIf::Receive to call Core::HandleUdp for generic UDP
  interception.
- Added raw buffer delivery overloads for InfraIf::SendUdp.

Test Implementation:
- Created test_1_4_DNS_TC_3.cpp which performs network formation,
  RA signaling (PIO/RIO/RDNSS), and DNS resolution triggers.
- Created verify_1_4_DNS_TC_3.py to validate network behavior,
  RA contents, and correct upstream query routing using pktverify.
- Integrated the new test into CMakeLists.txt and the default
  run_nexus_tests.sh suite.

tests/nexus/platform/nexus_node.cpp

@@ -40,6 +40,7 @@ void Node::Reset(void)
     mAlarmMilli.Reset();
     mAlarmMicro.Reset();
     mMdns.Reset();
+    mUpstreamDns.Reset();
     mInfraIf.mPendingTxQueue.DequeueAndFreeAll();
     mPendingTasklet = false;