call_on_each_event: Do not pass heartbeat events to clients.

Heartbeat events are intended to be an invisible part of the Zulip
longpolling protocol, not an event that an application should every
process, so it makes sense to exclude them from the events we pass to
this application.

Reproducer: https://github.com/showell/zulip-api-examples/blob/main/heartbeat_bug.py.

See https://chat.zulip.org/#narrow/stream/137-feedback/topic/api.20client.20silent.20failure
for more context.

zulip/zulip/__init__.py

@@ -819,6 +819,15 @@ class Client:
 
             for event in res["events"]:
                 last_event_id = max(last_event_id, int(event["id"]))
+
+                if event["type"] == "heartbeat":
+                    # Heartbeat events are sent to clients regardless
+                    # of the client's requested event types, and are
+                    # intended to be an internal part of the Zulip
+                    # longpolling protocol, not something that clients
+                    # need to handle.
+                    continue
+
                 callback(event)
 
     def call_on_each_message(