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/ObjectenGatewayTests.cs

@@ -83,6 +83,43 @@ public class ObjectenGatewayTests
 
     private static RegisterRecord Record() => new("zaak-uuid-1", RegisterRecordStatus.Ingeschreven, "REG-2026-0001");
 
+    [Fact]
+    public async Task Reads_a_register_record_back_from_its_object_url(/* S-19b-2 */)
+    {
+        var sent = new List<Sent>();
+        var objectUrl = new Uri("http://objecten:8000/api/v2/objects/obj-9");
+        var gateway = Gateway(sent, _ => Json(new
+        {
+            url = objectUrl.ToString(),
+            record = new { data = new { id = "zaak-uuid-1", status = "INGESCHREVEN", reference = "REG-2026-0001" } },
+        }));
+
+        var record = await gateway.GetAsync(objectUrl);
+
+        // The object is fetched directly by the URL the notification carried — no objecttype
+        // resolution and no search, unlike a write.
+        var read = Assert.Single(sent);
+        Assert.Equal(HttpMethod.Get, read.Method);
+        Assert.Equal(objectUrl, read.Uri);
+        // Objecten is a geo API: the CRS header is required on reads too.
+        Assert.Equal("EPSG:4326", read.AcceptCrs);
+        Assert.Equal("Token objecten-token", read.Auth);
+        Assert.Equal("zaak-uuid-1", record!.Id);
+        Assert.Equal("INGESCHREVEN", record.Status);
+        Assert.Equal("REG-2026-0001", record.Reference);
+    }
+
+    [Fact]
+    public async Task Reading_an_object_that_is_gone_yields_no_record(/* S-19b-2 */)
+    {
+        var sent = new List<Sent>();
+        var gateway = Gateway(sent, _ => new HttpResponseMessage(HttpStatusCode.NotFound));
+
+        // A record deleted between the notification and the read is not an error — there is simply
+        // nothing to project (§8.6: the subscriber tolerates whatever order deliveries arrive in).
+        Assert.Null(await gateway.GetAsync(new Uri("http://objecten:8000/api/v2/objects/gone")));
+    }
+
     [Fact]
     public async Task Creates_the_object_when_none_exists_for_the_registration()
     {