fix(middleware): preserve request bodies beyond the dump limit

[jakezwang]

MaxRequestBytes limits the request data captured by BodyDump, but the middleware also drains and discards everything after that limit. As a result, the next handler receives a truncated request body, even though Content-Length still describes the full request.

Replay the captured prefix followed by the unread body, and forward Close to the original body. The dump remains bounded. Tests cover full delivery beyond the limit, errors while reading the remainder, and body ownership.

Validation: go test -race ./..., go vet ./....