[v1.x] Backport #1912 and cut a release: a non-UTF-8 POST body on Streamable HTTP gets HTTP 500 INTERNAL_ERROR plus an ERROR cascade instead of the HTTP 400 PARSE_ERROR; the parse broadening ships only on main and the 2.0.0 pre-releases

[v1.x] Backport #1912 and cut a release: a non-UTF-8 POST body on Streamable HTTP gets HTTP 500 INTERNAL_ERROR plus an ERROR cascade instead of the HTTP 400 PARSE_ERROR; the parse broadening ships only on main and the 2.0.0 pre-releases #5518