port forward: the connect guard counts a live tunnel as connected

`connect_port_forward_if_needed` returned early only for a raw-pipe
socket; called again with a tunnel up it would have built a second
`PortForwardMux` and dropped every channel of the first. Not reachable
today, since the logon response is sent once, but the other checks in
this change already read `is_port_forward()`.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

src/server/connection.rs

@@ -1661,7 +1661,7 @@ impl Connection {
     }
 
     async fn connect_port_forward_if_needed(&mut self) -> bool {
-        if self.port_forward_socket.is_some() {
+        if self.is_port_forward() {
             return true;
         }
         let Some(login_request::Union::PortForward(pf)) = self.lr.union.as_ref() else {