fix(verify): attach stdin (-i) so the zaak-status read-back runs (refs #106)

The cancellation reaches OpenZaak correctly (Geannuleerd status_created), but
the verify read-back ran 'docker run python:3-slim python -' without -i, so the
heredoc never reached the container's stdin and the script was empty — the check
always saw an empty status. Add -i so the read actually queries OpenZaak.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infra/run-domain-check.sh

@@ -364,7 +364,8 @@ zaak_url_v="$(printf '%s' "$body" | grep -oiE 'http://[^"]*/zaken/api/v1/zaken/[
 echo ">> confirming the zaak $zaak_url_v reached the Geannuleerd status in OpenZaak"
 
 read_zaak_status() {
-  docker run --rm --network "$net" \
+  # -i so the heredoc reaches `python -` on the container's stdin (without it the script is empty).
+  docker run --rm -i --network "$net" \
     -e OZ_CLIENT_ID="${OZ_CLIENT_ID:-big-reference-seed}" \
     -e OZ_SECRET="${OZ_SECRET:-insecure-dev-secret-change-me}" \
     python:3-slim python - "$1" <<'PY'

services/acl/Acl.Tests/OpenZaakGatewayTests.cs

@@ -310,14 +310,12 @@ public class OpenZaakGatewayTests
         Assert.Contains("\"zaak\":\"" + ZaakUrl + "\"", resultaat.Body);
         // The cancellation resultaat (Vervallen) is chosen by name — not the approval one (Geregistreerd).
         Assert.Contains("\"resultaattype\":\"http://openzaak/catalogi/api/v1/resultaattypen/vervallen\"", resultaat.Body);
-        Assert.True(resultaat.Length > 0);
 
         var status = rec.Sent("/statussen");
         Assert.Contains("\"zaak\":\"" + ZaakUrl + "\"", status.Body);
         // The Geannuleerd statustype is chosen by name — not the Afgehandeld eindstatus (approval).
         Assert.Contains("\"statustype\":\"http://openzaak/catalogi/api/v1/statustypen/geannuleerd\"", status.Body);
         Assert.Contains("\"datumStatusGezet\":\"2026-06-04T00:00:00Z\"", status.Body);
-        Assert.True(status.Length > 0);
     }
 
     [Fact]
@@ -336,6 +334,56 @@ public class OpenZaakGatewayTests
         Assert.Contains("Geannuleerd", ex.Message);
     }
 
+    [Fact]
+    public async Task Cancelling_rejects_a_null_zaak_without_calling_openzaak()
+    {
+        var handler = new StubHandler(_ => throw new InvalidOperationException("should not be sent"));
+        await Assert.ThrowsAsync<ArgumentNullException>(() =>
+            Gateway(handler).SetZaakToCancellationStatusAsync(null!, Zaaktype, new DateOnly(2026, 6, 4)));
+    }
+
+    [Fact]
+    public async Task Cancelling_rejects_a_null_zaaktype_without_calling_openzaak()
+    {
+        var handler = new StubHandler(_ => throw new InvalidOperationException("should not be sent"));
+        await Assert.ThrowsAsync<ArgumentNullException>(() =>
+            Gateway(handler).SetZaakToCancellationStatusAsync(new Uri(ZaakUrl), null!, new DateOnly(2026, 6, 4)));
+    }
+
+    [Fact]
+    public async Task Cancelling_surfaces_the_failure_when_recording_the_resultaat_is_rejected()
+    {
+        var rec = new Recorder();
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(() =>
+            Gateway(ApprovalStub(rec, new OzRoutes
+            {
+                StatustypenJson = CancellationStatustypenJson,
+                ResultaattypenJson = CancellationResultaattypenJson,
+                ResultaatPostStatus = HttpStatusCode.BadRequest,
+            })).SetZaakToCancellationStatusAsync(new Uri(ZaakUrl), Zaaktype, new DateOnly(2026, 6, 4)));
+
+        Assert.Contains("cancellation resultaat", ex.Message);
+        // It fails on the resultaat, before it ever posts the status.
+        Assert.Equal(-1, rec.IndexOf("/statussen"));
+    }
+
+    [Fact]
+    public async Task Cancelling_surfaces_the_failure_when_recording_the_status_is_rejected()
+    {
+        var rec = new Recorder();
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(() =>
+            Gateway(ApprovalStub(rec, new OzRoutes
+            {
+                StatustypenJson = CancellationStatustypenJson,
+                ResultaattypenJson = CancellationResultaattypenJson,
+                StatusPostStatus = HttpStatusCode.BadRequest,
+            })).SetZaakToCancellationStatusAsync(new Uri(ZaakUrl), Zaaktype, new DateOnly(2026, 6, 4)));
+
+        Assert.Contains("cancellation status", ex.Message);
+    }
+
     [Fact]
     public async Task Approving_falls_back_to_the_highest_volgnummer_when_no_eindstatus_is_flagged()
     {