create_client now returns 'id' (was 'client_id'), get_client returns a flat
record with balance as a field (was nested under 'client'), and list_clients
is implemented returning {'clients': [...]} with the same flat shape. A
cross-method shape-consistency test (create/get/list key-set agreement)
plus a wire-shape test mirroring the server's _client_out allow-list guard
against future drift. server.py/test_server.py untouched (issue #2/!13).
