feat(acl): set a zaak to its eindstatus via ZGW + POST /statussen endpoint (refs #75)

The gateway resolves the zaaktype's eindstatus from the catalogus (isEindstatus,
falling back to the highest volgnummer) and POSTs a status against the zaak. Exposed
as POST /statussen for the domain's approve use case. Adds an integration test that
sets the eindstatus against a real OpenZaak and verifies the zaak's current status.

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

services/acl/Acl.Infrastructure/OpenZaakGateway.cs

@@ -41,6 +41,56 @@ public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) :
         return new Uri(created.Url);
     }
 
+    public async Task SetZaakToEindstatusAsync(Uri zaakUrl, Uri zaaktypeUrl, DateOnly datumStatusGezet, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+        ArgumentNullException.ThrowIfNull(zaaktypeUrl);
+
+        var eindstatus = await ResolveEindstatusAsync(zaaktypeUrl, ct);
+
+        using var message = new HttpRequestMessage(
+            HttpMethod.Post, new Uri(options.BaseUrl, "/zaken/api/v1/statussen"))
+        {
+            Content = JsonContent.Create(new StatusDto(
+                zaakUrl.ToString(),
+                eindstatus.ToString(),
+                // datumStatusGezet is a ZGW date-time; set it at the start of the given day (UTC).
+                datumStatusGezet.ToDateTime(TimeOnly.MinValue, DateTimeKind.Utc).ToString("yyyy-MM-ddTHH:mm:ssZ"))),
+        };
+        message.Headers.Authorization =
+            new AuthenticationHeaderValue("Bearer", ZgwToken.Mint(options.ClientId, options.Secret));
+        // The status resource carries no geometry, so no CRS headers. As with zaak-create, buffer the
+        // body so uwsgi gets a Content-Length instead of a chunked body.
+        await message.Content.LoadIntoBufferAsync(ct);
+
+        using var response = await http.SendAsync(message, ct);
+        response.EnsureSuccessStatusCode();
+    }
+
+    /// <summary>Resolve the zaaktype's eindstatus (the terminal statustype) from the catalogus.</summary>
+    private async Task<Uri> ResolveEindstatusAsync(Uri zaaktypeUrl, CancellationToken ct)
+    {
+        var query = new Uri(options.BaseUrl,
+            "/catalogi/api/v1/statustypen?status=alles&zaaktype=" + Uri.EscapeDataString(zaaktypeUrl.ToString()));
+        using var message = new HttpRequestMessage(HttpMethod.Get, query);
+        message.Headers.Authorization =
+            new AuthenticationHeaderValue("Bearer", ZgwToken.Mint(options.ClientId, options.Secret));
+
+        using var response = await http.SendAsync(message, ct);
+        response.EnsureSuccessStatusCode();
+
+        var page = await response.Content.ReadFromJsonAsync<StatustypePage>(ct)
+            ?? throw new InvalidOperationException("OpenZaak returned an empty statustypen response");
+        var results = page.Results ?? [];
+
+        // OpenZaak flags the terminal statustype (highest volgnummer) as isEindstatus; fall back to the
+        // highest volgnummer if the flag is absent.
+        var eindstatus = results.FirstOrDefault(s => s.IsEindstatus)
+            ?? results.OrderByDescending(s => s.Volgnummer).FirstOrDefault()
+            ?? throw new InvalidOperationException($"No statustypen found for zaaktype {zaaktypeUrl}");
+        return new Uri(eindstatus.Url);
+    }
+
     private sealed record ZaakDto(
         [property: JsonPropertyName("bronorganisatie")] string Bronorganisatie,
         [property: JsonPropertyName("zaaktype")] string Zaaktype,
@@ -50,4 +100,17 @@ public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) :
 
     private sealed record ZaakCreatedDto(
         [property: JsonPropertyName("url")] string Url);
+
+    private sealed record StatusDto(
+        [property: JsonPropertyName("zaak")] string Zaak,
+        [property: JsonPropertyName("statustype")] string Statustype,
+        [property: JsonPropertyName("datumStatusGezet")] string DatumStatusGezet);
+
+    private sealed record StatustypePage(
+        [property: JsonPropertyName("results")] IReadOnlyList<StatustypeDto>? Results);
+
+    private sealed record StatustypeDto(
+        [property: JsonPropertyName("url")] string Url,
+        [property: JsonPropertyName("volgnummer")] int Volgnummer,
+        [property: JsonPropertyName("isEindstatus")] bool IsEindstatus);
 }