Updates the fields shared by every activity type (name and lifecycle state), regardless of whether the activity is a trigger, target or action. For type-specific payloads use the type's own endpoint (e.g. Update Send mail action). This was already implemented and in use; this documents the existing behavior.'name' must be present in the body (send null to clear it); 'state' is optional (send null to clear it, or 'paused'/'deleted'). Note: despite being a lookup-failure, a missing activity currently returns 400, not 404.