test(acl): integration test opens a real zaak against OpenZaak (refs #46)

S-04a: the deferred S-04 acceptance criterion. A gated Acl.IntegrationTests
project (Category=Integration) drives the real OpenZaakGateway against the
running compose stack — real ZGW JWT auth and the real POST /zaken contract a
stubbed HttpMessageHandler cannot exercise. The lane is kept out of the fast
checks: make unit filters Category!=Integration, Stryker is pinned to Acl.Tests,
and a new make integration target brings the stack up, seeds a published zaaktype
and tears down.

Red: against real OpenZaak the gateway POST fails 400 — JsonContent streams the
body chunked and OpenZaak's uwsgi rejects it. Fixed in the next commit.

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

services/acl/stryker-config.json

@@ -1,6 +1,7 @@
 {
   "stryker-config": {
     "solution": "Acl.slnx",
+    "test-projects": ["Acl.Tests/Acl.Tests.csproj"],
     "reporters": ["progress", "html"],
     "thresholds": {
       "high": 95,