fix(infra): reach Objecten by service name in the register-record check (refs #149)

CI caught my own check falling into the constraint ADR-0028 documents: it looked
Objecttypen up by container IP, so the objecttype URL came back IP-addressed and
Objecten rejected it as "not one of the available choices". Reach both by
service name — compose DNS resolves them, and neither request has OpenZaak's
URL-validity constraint that made IPs necessary elsewhere in this script.

The 400 also spent the full 60s timeout disguised as "transport:" because
HTTPError is a URLError subclass. Handle it separately: a 4xx now fails
immediately with the response body, which is where the real reason was.

Verified both ways against a live Objecten: absent record → exit 1 with the
reason, present record → exit 0.

infra/run-domain-check.sh

@@ -146,15 +146,21 @@ echo "OK — behandelaar claimed and completed the Beoordelen task; the registra
 # Assert it for THIS registration (matched on its reference) rather than "some INGESCHREVEN record":
 # the shared verify stack carries records from earlier runs. The container-name filters are anchored
 # on the compose replica suffix so they don't also match objecten-db / objecttypen-db.
+#
+# Unlike every other check here, these two are reached by SERVICE NAME, not container IP. Objecttypen
+# echoes the request Host into the objecttype `url`, and Objecten only accepts the objecttype URL that
+# matches its configured api_root (http://objecttypen:8000/api/v2/) — an IP-addressed lookup yields a
+# URL Objecten rejects with 400 (ADR-0028). Compose DNS resolves both names on this network, and
+# neither request has OpenZaak's URL-validity constraint.
 echo ">> asserting the approval wrote the register record to Objecten (S-19a)"
 obj="$(docker ps -q --filter 'name=objecten[-_][0-9]+$' | head -1)"
 objt="$(docker ps -q --filter 'name=objecttypen[-_][0-9]+$' | head -1)"
 [ -n "$obj" ] || { echo "FAIL — no running objecten container" >&2; exit 1; }
 [ -n "$objt" ] || { echo "FAIL — no running objecttypen container" >&2; exit 1; }
 rr="$(docker create --network "$net" \
-  -e "OBJECTEN=http://$(ip "$obj"):8000" \
+  -e "OBJECTEN=http://objecten:8000" \
   -e "OBJECTEN_TOKEN=${OBJECTEN_TOKEN:-1234567890abcdef1234567890abcdef12345678}" \
-  -e "OBJECTTYPEN=http://$(ip "$objt"):8000" \
+  -e "OBJECTTYPEN=http://objecttypen:8000" \
   -e "OBJECTTYPEN_TOKEN=${OBJECTTYPEN_TOKEN:-0123456789abcdef0123456789abcdef01234567}" \
   -e "REGISTRATION_REFERENCE=$reg_id" \
   python:3-slim python /register-record-check.py)"