MINOR: h3: abort request if not completed before full response

MINOR: h3: abort request if not completed before full response

A HTTP server may provide a complete response even prior receiving the
full request. In this case, RFC 9114 allows the server to abort read
with a STOP_SENDING with error code H3_NO_ERROR.

This scenario was notably reproduced with haproxy and an inactive
server. If the client send a POST request, haproxy may provide a full
HTTP 503 response before the end of the full request.