subscription: remove payload on websocket's Ping (#4853)

Removes invalid payload on graphql-ws Ping message

Fixes #4852

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**Checklist**

Complete the checklist (and note appropriate exceptions) before the PR
is marked ready-for-review.

- [x] Changes are compatible[^1]
- [x] Documentation[^2] completed
- [x] Performance impact assessed and acceptable
- Tests added and passing[^3]
    - [x] Unit Tests
    - [ ] Integration Tests
    - [ ] Manual Tests

**Exceptions**

*Note any exceptions here*

**Notes**

[^1]: It may be appropriate to bring upcoming changes to the attention
of other (impacted) groups. Please endeavour to do this before seeking
PR approval. The mechanism for doing this will vary considerably, so use
your judgement as to how and when to do this.
[^2]: Configuration is an important part of many changes. Where
applicable please try to document configuration examples.
[^3]: Tick whichever testing boxes are applicable. If you are adding
Manual Tests, please document the manual testing (extensively) in the
Exceptions.

.changesets/fix_i1g_subscription_websocket_fix_ping.md

@@ -0,0 +1,6 @@
+### Remove invalid payload on graphql-ws Ping message ([Issue #4852](https://github.com/apollographql/router/issues/4852))
+
+According to [graphql-ws spec](https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md#ping) `Ping` payload should be an object or null but router was sending a string.
+To ensure better compatibility Ping's payload was removed. 
+
+By [@IvanGoncharov](https://github.com/IvanGoncharov) in https://github.com/apollographql/router/pull/4852

apollo-router/src/protocols/websocket.rs

@@ -416,13 +416,7 @@ where
                     tokio::time::interval_at(tokio::time::Instant::now() + duration, duration);
                 interval.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
                 let mut heartbeat_stream = IntervalStream::new(interval)
-                    .map(|_| {
-                        Ok(ClientMessage::Ping {
-                            payload: Some(serde_json_bytes::Value::String(
-                                "APOLLO_ROUTER_HEARTBEAT".into(),
-                            )),
-                        })
-                    })
+                    .map(|_| Ok(ClientMessage::Ping { payload: None }))
                     .take_until(close_sentinel);
                 if let Err(err) = sink.send_all(&mut heartbeat_stream).await {
                     tracing::trace!("cannot send heartbeat: {err:?}");
@@ -755,7 +749,7 @@ mod tests {
                    tokio::time::sleep(duration).await;
                    let ping_message = socket.next().await.unwrap().unwrap();
                    assert_eq!(ping_message, AxumWsMessage::Text(
-                       serde_json::to_string(&ClientMessage::Ping { payload: Some(serde_json_bytes::Value::String("APOLLO_ROUTER_HEARTBEAT".into())) }).unwrap(),
+                       serde_json::to_string(&ClientMessage::Ping { payload: None }).unwrap(),
                    ));
 
                    assert!(