fix(connlib): send dns proxy ips even with overlapping internet resource (#5902)

To determine whether we send proxy IPs we depend on the `allowed_ips`,
since that's where we track what resources we have sent to a given
gateway.

However, the way we were matching if a given resource destination was
sent was using `longest_match` and with overlapping DNS this no longer
works, since this will match for internet resources even if the proxy IP
wasn't sent.

So we check that it's a DNS resource and if it's we exactly match on the
allowed ip table.

Alternatively, we could keep track of `sent_ips` for a gateway, though
this is a bit of a redundant state that we need to keep in sync but has
the benefit of being more explicit, so I'm open to do that in a follow
up PR. But I'd like to merge this to get ready for internet resources.

rust/connlib/tunnel/src/client.rs

@@ -384,6 +384,13 @@ impl ClientState {
         })
     }
 
+    fn is_dns_resource(&self, resource: &ResourceId) -> bool {
+        matches!(
+            self.resources_by_id.get(resource),
+            Some(ResourceDescription::Dns(_))
+        )
+    }
+
     pub(crate) fn encapsulate<'s>(
         &'s mut self,
         packet: MutableIpPacket<'_>,
@@ -406,6 +413,9 @@ impl ClientState {
             return None;
         };
 
+        // We read this here to prevent problems with the borrow checker
+        let is_dns_resource = self.is_dns_resource(&resource);
+
         let Some(peer) = peer_by_resource_mut(&self.resources_gateways, &mut self.peers, resource)
         else {
             self.on_not_connected_resource(resource, &dst, now);
@@ -419,7 +429,9 @@ impl ClientState {
             now,
         );
 
-        if peer.allowed_ips.longest_match(dst).is_none() {
+        // Allowed IPs will track the IPs that we have sent to the gateway along with a list of ResourceIds
+        // for DNS resource we will send the IP one at a time.
+        if is_dns_resource && peer.allowed_ips.exact_match(dst).is_none() {
             let gateway_id = peer.id();
             self.send_proxy_ips(&dst, resource, gateway_id);
             return None;