The mint route shipped without tests while the create route beside it
has three, so the two properties that make it a backfill rather than a
second creation route were unasserted: that it refuses before queuing,
and that it queues the mint node and nothing else.
Queuing a config-repo scaffold or a deploy against an agent that already
exists is the failure the second of those catches, and it is invisible
from the status code -- a version that queued the whole creation graph
would answer 200 with a node id just the same.
The agent name gets its own arm. create_agent's hive is checked against
the swarm roster, which incidentally rejects a name that is not an
identifier; an agent name has no roster to check against, so the parse
is the only thing between a traversal and a store path built out of it.
MintAgentIdentityResponse derives Clone + Debug to match
CreateAgentResponse -- expect_err on the refusal arms needs Debug on the
success type.
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