test(acl): submit writes an INGEDIEND record, and records are readable back (refs #153)

Ports and failing tests for the ACL half of S-19b-2, ahead of the implementation.

Once the projection is sourced from Objecten (ADR-0028's stated direction), a submitted
registration has to exist in the register the moment the zaak is opened — otherwise
re-sourcing silently drops every INGEDIEND row, since today only approval writes a record.
So `OpenZaakAsync` gains a second write, and approval upserts that same record to
INGESCHREVEN.

The subscriber gets only an object URL on an `objecten` notification (the payload carries no
record data) and may not read Objecten itself (§8.1), so `IRegisterRecordGateway` gains a
read and `AclService` exposes it.

Red:
- AclService does not yet write on open → the record assertion fails on an empty list.
- ObjectenGateway.GetAsync is a shell throwing NotImplementedException; its tests pin the
  contract: fetch the object URL directly (no objecttype resolution, no search), the CRS
  header a geo API requires, static Token auth, and a 404 read as "nothing to project"
  rather than an error (§8.6).

services/acl/Acl.Tests/AclServiceTests.cs

@@ -79,11 +79,21 @@ public class AclServiceTests
     {
         public readonly List<RegisterRecord> Upserted = [];
 
+        public RegisterRecord? Stored;
+
+        public Uri? ReadFrom;
+
         public Task UpsertAsync(RegisterRecord record, CancellationToken ct = default)
         {
             Upserted.Add(record);
             return Task.CompletedTask;
         }
+
+        public Task<RegisterRecord?> GetAsync(Uri objectUrl, CancellationToken ct = default)
+        {
+            ReadFrom = objectUrl;
+            return Task.FromResult(Stored);
+        }
     }
 
     private static AclDefaults Defaults() => new()
@@ -130,6 +140,52 @@ public class AclServiceTests
         Assert.Equal("reg-77", req.Identificatie);
     }
 
+    [Fact]
+    public async Task Opening_a_zaak_also_writes_an_ingediend_register_record(/* S-19b-2 */)
+    {
+        var gateway = new FakeGateway();
+        var register = new FakeRegisterRecordGateway();
+        var service = ServiceWith(gateway, register, Defaults(), new DateOnly(2026, 6, 4));
+
+        await service.OpenZaakAsync(new DomainRegistration("123456782", "reg-77"));
+
+        // The register — not ZGW — is what the read projection is sourced from (ADR-0028), so a
+        // submitted registration has to exist there the moment the zaak is opened, not only on
+        // approval. Approval upserts this same record to INGESCHREVEN.
+        var record = Assert.Single(register.Upserted);
+        Assert.Equal("abc", record.Id);
+        Assert.Equal("INGEDIEND", record.Status);
+        // The reference comes from the registration itself — no ZGW read-back needed on this path.
+        Assert.Equal("reg-77", record.Reference);
+    }
+
+    [Fact]
+    public async Task Reading_a_register_record_goes_through_the_objecten_gateway(/* S-19b-2 */)
+    {
+        var gateway = new FakeGateway();
+        var register = new FakeRegisterRecordGateway { Stored = new RegisterRecord("abc", "INGESCHREVEN", "reg-77") };
+        var service = ServiceWith(gateway, register, Defaults(), new DateOnly(2026, 6, 4));
+        var objectUrl = new Uri("http://objecten.local:8000/api/v2/objects/9de4a2ca");
+
+        var record = await service.GetRegisterRecordAsync(objectUrl);
+
+        Assert.Equal(objectUrl, register.ReadFrom);
+        Assert.Equal("abc", record!.Id);
+        Assert.Equal("INGESCHREVEN", record.Status);
+        Assert.Equal("reg-77", record.Reference);
+    }
+
+    [Fact]
+    public async Task Reading_a_register_record_from_a_null_url_is_rejected(/* S-19b-2 */)
+    {
+        var gateway = new FakeGateway();
+        var register = new FakeRegisterRecordGateway();
+        var service = ServiceWith(gateway, register, Defaults(), new DateOnly(2026, 6, 4));
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => service.GetRegisterRecordAsync(null!));
+        Assert.Null(register.ReadFrom);
+    }
+
     [Fact]
     public async Task Opening_a_zaak_reflects_a_default_fill_update(/* S-15b */)
     {