The discovery filter read one byte past its 1 MiB cap to detect a body too
large to rewrite, then closed the upstream body and forwarded the buffer it
had — the response reached the client truncated at exactly the cap, with
Content-Length rewritten to match so nothing looked wrong until the client
tried to parse it.
Splice the bytes already read back in front of the unread remainder and
forward the response as the upstream sent it, headers untouched.