test(acl): cover the resolve null/error paths to hold the mutation ratchet (refs #113)

Kills the survivors the new resolution code introduced: null Results (no "results"
in the response) must throw "none found" not NRE; a non-success catalogi response
must surface an error naming the resource. ACL mutation score 93.89% (break 90%).

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

services/acl/Acl.Tests/OpenZaakGatewayTests.cs

@@ -721,7 +721,11 @@ public class OpenZaakGatewayTests
     [Fact]
     public async Task Resolves_the_informatieobjecttype_url_by_omschrijving()
     {
-        var handler = new StubHandler(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+        HttpRequestMessage? seen = null;
+        var handler = new StubHandler(req =>
+        {
+            seen = req;
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
             {
                 Content = JsonContent.Create(new
                 {
@@ -731,14 +735,71 @@ public class OpenZaakGatewayTests
                         new { url = "http://openzaak/catalogi/api/v1/informatieobjecttypen/dip", omschrijving = "Diploma" },
                     },
                 }),
-        }));
+            });
+        });
 
         var url = await Gateway(handler).ResolveInformatieobjecttypeUrlAsync("Diploma");
 
-        // Matches on omschrijving, not position.
+        // Queries the published informatieobjecttypen collection, and matches on omschrijving (not position).
+        Assert.Contains("/catalogi/api/v1/informatieobjecttypen", seen!.RequestUri!.ToString());
+        Assert.Contains("status=definitief", seen.RequestUri!.Query);
         Assert.Equal("http://openzaak/catalogi/api/v1/informatieobjecttypen/dip", url.ToString());
     }
 
+    [Fact]
+    public async Task Resolving_a_zaaktype_throws_when_the_response_carries_no_results()
+    {
+        // No "results" property → the page's Results is null; the gateway must treat that as "none
+        // found" (not dereference null).
+        var handler = new StubHandler(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = JsonContent.Create(new { count = 0 }),
+        }));
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => Gateway(handler).ResolveZaaktypeUrlAsync("BIG-REGISTRATIE"));
+    }
+
+    [Fact]
+    public async Task Resolving_an_informatieobjecttype_throws_when_the_response_carries_no_results()
+    {
+        var handler = new StubHandler(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = JsonContent.Create(new { count = 0 }),
+        }));
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => Gateway(handler).ResolveInformatieobjecttypeUrlAsync("Diploma"));
+    }
+
+    [Fact]
+    public async Task Resolving_a_zaaktype_surfaces_a_non_success_catalogi_response()
+    {
+        var handler = new StubHandler(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.InternalServerError)
+        {
+            Content = new StringContent("boom"),
+        }));
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(
+            () => Gateway(handler).ResolveZaaktypeUrlAsync("BIG-REGISTRATIE"));
+        // The error names the resource being queried and includes OpenZaak's body.
+        Assert.Contains("zaaktypen", ex.Message);
+        Assert.Contains("boom", ex.Message);
+    }
+
+    [Fact]
+    public async Task Resolving_an_informatieobjecttype_surfaces_a_non_success_catalogi_response()
+    {
+        var handler = new StubHandler(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.InternalServerError)
+        {
+            Content = new StringContent("boom"),
+        }));
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(
+            () => Gateway(handler).ResolveInformatieobjecttypeUrlAsync("Diploma"));
+        Assert.Contains("informatieobjecttypen", ex.Message);
+    }
+
     [Fact]
     public async Task Resolving_an_informatieobjecttype_throws_when_no_omschrijving_matches()
     {