Return events after the named consumer's server-held checkpoint. If the
consumer has no checkpoint, reading begins at the oldest event still
inside retention. Omitting consumer uses the default consumer.

Fetching is read-only. Repeating this request before acknowledgement can
return the same events. Process the complete batch, then send the response
cursor to the acknowledge endpoint with the same consumer name. The
next scheduled fetch does not need to retain or submit that cursor.

If a stored checkpoint has fallen outside retention, reading resumes at
the oldest retained event and retention.gap_detected is true. Session
events are webhook-only and are never returned.