Updates one or more metadata attributes of an existing audience without requiring a full replacement. Supported fields: name, folderId, description, and notes. Only fields present in the request body are modified; all other attributes remain unchanged. Use this endpoint to rename an audience, move it to a different folder, or update its description or notes. Returns 200 with the updated audience resource. Returns 400 if the request body is malformed or contains unsupported fields. Returns 404 if no audience matches the provided audienceId.