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/register-record-check.py

@@ -77,6 +77,14 @@ def main():
             if ok:
                 print(f"OK — approval wrote the register record to Objecten: {detail}")
                 return 0
+        except urllib.error.HTTPError as e:
+            # A 4xx is us, not a cold start — retrying just hides the reason until the deadline.
+            # (A rejected objecttype URL shows up here as a 400 with a very specific body.)
+            body = e.read().decode(errors="replace")[:400]
+            if e.code < 500:
+                print(f"FAIL — HTTP {e.code} from {e.url}: {body}", file=sys.stderr)
+                return 1
+            detail = f"HTTP {e.code}: {body}"
         except (urllib.error.URLError, ConnectionError, TimeoutError) as e:
             detail = f"transport: {e}"
         time.sleep(3)