Prior coverage only exercised getThumbnailUrl() as a Java method call. The new case serialises via ObjectMapper and asserts the resulting JSON contains "thumbnailUrl":"..." so we catch silent breakages in the wire contract (getter rename, @JsonIgnore, visibility drop) — not just regressions in the method's return value (#309). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>