fix(acl): buffer the zaak POST body so OpenZaak accepts it (refs #46)

OpenZaak runs behind uwsgi, which rejects a chunked request body with 400.
JsonContent streams without a Content-Length (Transfer-Encoding: chunked), so
buffer it first. Only a real OpenZaak surfaces this — the integration test from
the previous commit now passes. A unit test asserts a Content-Length is sent
(captured before the stub reads/buffers the body), guarding the fix in the fast
lane and killing the Stryker mutant that would otherwise survive.

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

services/acl/Acl.Infrastructure/OpenZaakGateway.cs

@@ -27,6 +27,11 @@ public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) :
         // ZRC is a geo API; it requires the CRS headers.
         message.Headers.Add("Accept-Crs", "EPSG:4326");
         message.Content.Headers.Add("Content-Crs", "EPSG:4326");
+        // OpenZaak runs behind uwsgi, which rejects a chunked request body with 400.
+        // JsonContent streams without a known length (→ Transfer-Encoding: chunked),
+        // so buffer it first to send a Content-Length instead. Only a real OpenZaak
+        // surfaces this — a stubbed HttpMessageHandler accepts either framing.
+        await message.Content.LoadIntoBufferAsync(ct);
 
         using var response = await http.SendAsync(message, ct);
         response.EnsureSuccessStatusCode();