Don't drop hibernatable ws if pumping

We had a bug where we can register two pump() promises because the
second pump() starts before the promise continuation of the first
completes. This meant that when the first pump() continuation started
running, it could cancel the second pump() task, preventing outgoing
messages from being sent.

src/workerd/api/web-socket.c++

@@ -728,7 +728,18 @@ void WebSocket::ensurePumping(jsg::Lock& js) {
           reportError(js, KJ_EXCEPTION(DISCONNECTED, "WebSocket peer disconnected"));
         }
       } else if (native.closedIncoming && native.closedOutgoing) {
-        if (native.state.is<Accepted>()) {
+        if (awaitingHibernatableRelease()) {
+          // Because Hibernatable WebSocket events are dispatched at different times,
+          // we may have a situation where one event's pump() promise finishes (setting
+          // isPumping to false), but then this continuation doesn't get scheduled until after we
+          // start handling the next websocket event, which may call send()/close(), and result in
+          // another pump() promise being registered.
+          //
+          // When the first event continues execution, if we release the WebSocket without
+          // confirming it is not currently pumping, we can transition the WebSockets state from
+          // Accepted to Released. This will cancel the second events pump() promise.
+          tryReleaseNative(js);
+        } else if (native.state.is<Accepted>()) {
           // Native WebSocket no longer needed; release.
           native.state.init<Released>();
         } else if (native.state.is<Released>()) {