fix(gateway): re-join topic in phoenix-channel on error (#10397)

For whatever reason, we seem to sometimes lose the association with the
"room" we are meant to be in in order to send messages to the portal.
Without joining the right room, messages get dropped silently.

To fix this, we re-join the room on such errors. Long-term, this will be
fixed by ditching phoenix-channel in favor of simple HTTP requests.

Related: #9649

rust/gateway/src/eventloop.rs

@@ -652,6 +652,16 @@ async fn phoenix_channel_event_loop(
                     break;
                 }
             }
+            Either::Left((
+                Ok(phoenix_channel::Event::ErrorResponse {
+                    topic,
+                    res: phoenix_channel::ErrorReply::UnmatchedTopic,
+                    ..
+                }),
+                _,
+            )) => {
+                portal.join(topic, ());
+            }
             Either::Left((Ok(phoenix_channel::Event::ErrorResponse { topic, req_id, res }), _)) => {
                 tracing::warn!(%topic, %req_id, "Request failed: {res:?}");
             }