[nexus] update mDNS traffic to flow through infrastructure link (#12720)

Update mDNS traffic simulation in the Nexus platform to flow through
the simulated infrastructure link. This ensures mDNS packets are
automatically written to the PCAP file generated by the simulated
infrastructure link.

Changes:
- Wrap mDNS messages in UDP/IPv6 headers and enqueue them on the
  simulated infrastructure interface (InfraIf).
- Implement a new SendUdp overload in InfraIf that accepts a Message
  payload.
- Update InfraIf::Receive to intercept mDNS UDP packets (port 5353)
  and deliver them to the Mdns module.
- Remove the dedicated ProcessMdns loop and manual PendingTx list from
  Core and Mdns, consolidating traffic processing through InfraIf.
- Initialize Mdns with a reference to the Node to allow access to
  InfraIf.
- Add GetMulticastAddress static helper to Mdns for 'ff02::fb'.

tests/nexus/platform/nexus_node.cpp

@@ -188,7 +188,7 @@ exit:
 #if OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE
 void Node::GetTrelSockAddr(Ip6::SockAddr &aSockAddr) const
 {
-    aSockAddr.SetAddress(mMdns.mIfAddresses[0]);
+    aSockAddr.SetAddress(mInfraIf.GetLinkLocalAddress());
     aSockAddr.SetPort(mTrel.mUdpPort);
 }
 #endif