duplicate field name is 201

Expected A create that cannot produce the field the caller asked for is rejected, the way a create against a retired name already is. The programmatic name the server derived is in the response body.

Actual Two creates of the same display name:

POST /schema/Version/fields  'zzprobe 019 collide'  -> 201  sg_zzprobe_019_collide
POST /schema/Version/fields  'zzprobe 019 collide'  -> 201  sg_zzprobe_019_collide_1

The second is a different field. Neither response names it: the programmatic name is absent from the body and is only the last segment of links.self.

Deleting is one-way:

No listing shows a retired field, so the 400 names a collision the caller cannot see and cannot resolve over REST.

Reproduce

curl -sS -X POST "$SITE/api/v1/schema/Version/fields" \
  -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" \
  -d '{"data_type":"text","properties":[{"property_name":"name","value":"my field"}]}'
# 201. The name is only in links.self

# The identical call again
curl -sS -X POST "$SITE/api/v1/schema/Version/fields" \
  -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" \
  -d '{"data_type":"text","properties":[{"property_name":"name","value":"my field"}]}'
# 201 again, for sg_my_field_1

Impact A create that is retried, by a job that lost its response or by a client written to be idempotent, adds a field per attempt. Each is permanent from REST: DELETE retires it and burns the programmatic name, and nothing over the API lists what was burned or frees it. The recovery is a person emptying the Trash in the web interface. An idempotent ensure() has to read /schema/<Type>/fields and match before every write, and the response not naming the field it made is what makes even that awkward.

Proposed change Answer 409 when the display name is taken, naming the existing field. Return the derived programmatic name in the response body rather than only in links.self. Independently, expose retired fields on a listing so the schema_field_create() failed 400 is something a caller can act on.