Each Save and Load now derives tenant_id from the run GID and pins it
in the WHERE clause. A caller that supplies an ID from another tenant
fails closed instead of silently reading or overwriting cross-tenant
checkpoint data. Also rejects oversize checkpoints on load as a
read-side guard against a tampered or migrated row exceeding
MaxCheckpointBytes.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>