PUTting the full entity dict back to data/entities/{id} sends server-managed
fields (id, created_at, updated_at, project_id, entity_type_id, ...) that
newer Kitsu rejects with ParameterException. Build a partial payload with
only name/description/nb_frames/data — and only fetch/merge existing data
when the caller is actually updating custom_data.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>