Fix nRF52 AsyncUDP multicast TX/RX race condition causing garbled packets (#9765)

socketSendUDP() calls yield() while waiting for SEND_OK, allowing the
cooperative scheduler to run AsyncUDP::runOnce(). When runOnce() calls
parsePacket() on the same socket during an active send, the multicast
loopback packet arriving from the switch produces a garbled 8-byte RX
header (wrong source IP, wrong payload length), leading to protobuf
decode errors in UdpMulticastHandler.

Add a volatile isSending flag that writeTo() sets around endPacket()
(the only yield point). runOnce() checks this flag and skips
parsePacket() while a send is in progress. Loopback packets stay
buffered in the W5100S RX buffer and are read cleanly on the next
poll cycle.

Also propagate writeTo() return value in UdpMulticastHandler::onSend()
instead of unconditionally returning true.

Made-with: Cursor

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/udp/UdpMulticastHandler.h

@@ -105,8 +105,7 @@ class UdpMulticastHandler final
         LOG_DEBUG("Broadcasting packet over UDP (id=%u)", mp->id);
         uint8_t buffer[meshtastic_MeshPacket_size];
         size_t encodedLength = pb_encode_to_bytes(buffer, sizeof(buffer), &meshtastic_MeshPacket_msg, mp);
-        udp.writeTo(buffer, encodedLength, udpIpAddress, UDP_MULTICAST_DEFAUL_PORT);
-        return true;
+        return udp.writeTo(buffer, encodedLength, udpIpAddress, UDP_MULTICAST_DEFAUL_PORT);
     }
 
   private: