Release handlers on close to break session reference cycles

Handlers commonly capture a clone of their own Session, and the Session
owns its handlers, so a closed session was never freed. On shutdown,
take the on_close handler instead of cloning it and clear the request
and notification handlers after it runs.

Bump to 0.2.1.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/protocol.rs

@@ -450,3 +450,43 @@ async fn axum_adapter_serves_sessions_next_to_http_routes() {
     tcp.read_to_string(&mut body).await.unwrap();
     assert!(body.starts_with("HTTP/1.1 200") && body.ends_with("ok"), "{body}");
 }
+
+#[tokio::test]
+async fn closing_releases_handlers_that_capture_the_session() {
+    let client = connect(start_server().await).await;
+    let captured = Arc::new(());
+
+    // Handlers typically hold a clone of their own session; that must not keep
+    // the session (and everything the handlers capture) alive after close.
+    client
+        .on_request::<Silent, _>({
+            let session = client.clone();
+            let captured = captured.clone();
+            move |_, ()| {
+                let _ = (&session, &captured);
+                async { Ok(()) }
+            }
+        })
+        .await;
+    client
+        .on_close({
+            let session = client.clone();
+            let captured = captured.clone();
+            move || {
+                let _ = (&session, &captured);
+                async { Ok(()) }
+            }
+        })
+        .await;
+    assert_eq!(Arc::strong_count(&captured), 3);
+
+    client.close().await.unwrap();
+
+    tokio::time::timeout(WAIT, async {
+        while Arc::strong_count(&captured) > 1 {
+            tokio::time::sleep(Duration::from_millis(10)).await;
+        }
+    })
+    .await
+    .expect("handlers were not released after close");
+}