fix(connlib): use correct constant for truncating DNS responses (#7551)

In case an upstream DNS server responds with a payload that exceeds the
available buffer space of an IP packet, we need to truncate the
response. Currently, this truncation uses the **wrong** constant to
check for the maximum allowed length. Instead of the
`MAX_DATAGRAM_PAYLOAD`, we actually need to check against a limit that
is less than the MTU as the IP layer and the UDP layer both add an
overhead.

To fix this, we introduce such a constant and provide additional
documentation on the remaining ones to hopefully avoid future errors.

rust/gateway/src/main.rs

@@ -106,7 +106,7 @@ async fn try_main(cli: Cli, telemetry: &mut Telemetry) -> Result<ExitCode> {
 
     let (sender, receiver) = mpsc::channel::<Interface>(10);
 
-    let mut tun_device_manager = TunDeviceManager::new(ip_packet::PACKET_SIZE, cli.tun_threads)?;
+    let mut tun_device_manager = TunDeviceManager::new(ip_packet::MAX_IP_SIZE, cli.tun_threads)?;
     let tun = tun_device_manager.make_tun()?;
     tunnel.set_tun(Box::new(tun));