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>
