Demonstrates spec §13.5 / §7.2: a second submit with the same
(principal, idempotency_key) replays the original job.accepted
verbatim, but a third submit reusing the key with a different agent
raises DuplicateKeyError.
python examples/idempotent_retry/server.py # terminal 1
python examples/idempotent_retry/client.py # terminal 2Client exits 0 when the retry returns the original job_id and the
mutated submit raises DuplicateKeyError.