fix(http): keep the error body of a failed streaming request

send() fills HttpResponse::body on every path including failures; send_stream()
was the one entry point that dropped it. A non-2xx answer to a streaming request
is an error document, not an event stream: SseParser finds no event boundary in
it, emits nothing, and the bytes stay in its private buffer. Callers were left
with a status line and no reason.

Capture the body when the status is not 2xx. Events are still parsed and
dispatched exactly as before, and nothing is copied on a 2xx stream, so the
success path is byte-identical.

The copy is bounded by stream_error_body_limit (1 MiB) so a server answering 5xx
with an endless body cannot grow the buffer without limit. Truncation lives in an
exported append_within_limit, in the same spirit as parse_chunk_size_line, with
three unit tests for under, across and past the limit; a live test against
httpbin's /status/418 covers the wiring.

Author: yspbwx2010