The core writeJSON helpers streamed the body via json.NewEncoder(w).Encode
after WriteHeader, which forces net/http into chunked transfer encoding and
omits Content-Length. Buffer the marshaled body first, set an accurate
Content-Length, then write — so API (and health) JSON responses advertise
their exact size. writeMeasuredJSON gets the same header. Adds a test
asserting the header matches the on-wire body length.
Co-authored-by: Eve <eve@multica-ai.local>
Co-authored-by: multica-agent <github@multica.ai>
