Return an empty result when a successful, completed group essence response omits its message list instead of reporting a malformed response.
Keep upstream errors and incomplete pagination responses explicit, and cover omitted and null list variants with regression tests.
Fixes#320.
Translate group essence responses into the established OneBot shape instead of exposing the upstream digest payload.
Return validated sender, operator, timestamp, message identifier, and message segment fields for text, face, image, and video entries. Retain the authoritative message metadata in one transaction so returned identifiers can be used by later essence actions without partial cache state.
Reject malformed, cross-group, incomplete, or unknown digest entries, and surface pagination or upstream failures instead of returning truncated success results.
Fixes#300.
DrSmoothl