fix: a displaced port forward must not remove the session

Bailing hands the caller an error, and its error branch calls
`check_remove_session`. For a port forward that resolves to a removal: the
check keeps a session only when another *remote* connection shares its key, so
a session made of tunnels alone - the ordinary port forward case - loses its
`SESSIONS` entry while the connection that displaced this one is still live.
The next connection in that session then re-prompts, having lost the recent
session it should have been able to authenticate against.

Close and return instead, which is what every other kind's displacement
already does: the final `on_close("End", true)` sees `closed` and returns, so
nothing else changes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

src/server/connection.rs

@@ -1251,7 +1251,11 @@ impl Connection {
                     // place a displacement can still reach it.
                     Some(data) = rx_from_authed.recv() => {
                         if let ipc::Data::Displaced = data {
-                            bail!("displaced by a newer connection");
+                            // Closed here rather than bailed: the caller answers an error by
+                            // removing the session, and this session is precisely what carries
+                            // on - on the connection that took this one's place.
+                            self.on_close("displaced by a newer connection", false).await;
+                            return Ok(());
                         }
                     }
                     res = forward.next() => {