zone_record: write to the id in the path

[aberoham]

PUT /zone_record/{id} writes to the id in the body, not the id in the path.
The handler reads the record named in the URL, finds it, then hands the store a
merge that starts with the path id and spreads the body over it. A body id wins.
The call answers 200 and reports success for a record it never touched.

Two records in one zone, driven through the routes:

before: 7314 address=203.0.113.2
PUT /zone_record/7313 {id: 7314, address: 198.51.100.99} -> 200
after:  7313 address=203.0.113.1   named in the URL, untouched
after:  7314 address=198.51.100.99 never named in the URL

zone_record.PUT forks id to optional. The key reaches the handler. The
zone and nameserver schemas do not list it and answer 400 with "id" is not allowed. Both handlers had the same argument order, so this puts all four
entities on the order group and user already use.

The regression test asserts on the bystander's stored row, since the response
body looks correct either way. Pinning the status code would tie the test to a
schema that accepts the key, so it does not. It fails on main and passes here.
Neither zone nor nameserver had a route test for its update handler. The two
changed lines arrived uncovered. They have a test now. All three stores run
clean: mysql 398, json and toml the file-store subset.