test(acl): kill cancellation-path mutants to clear the mutation ratchet (refs #106)

The new SetZaakToCancellationStatusAsync added surviving mutants (uncovered
null-guards and the failure-action messages) that dropped the ACL mutation
score below the 90% break threshold. Add null-argument and rejected-POST tests
for the cancellation path; score back to 93.46%.

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

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()
     {