server: PEG until() accepts a missing final delimiter #39

Upstream: ggml-org#27772

A final, non-lenient PEG parse incorrectly succeeds when until() reaches end-of-input without finding its delimiter.

Environment: current main 67672dc5b, Linux aarch64.

auto parser = build_peg_parser([](common_peg_parser_builder & p) {
    return p.until("\"");
});
common_peg_parse_context ctx("no closing quote anywhere in here");
auto result = parser.parse(ctx);

Observed: result.success() is true; the added regression assertion expecting false fails. This can let a truncated Gemma 4 tool argument pass the final parse.

common_peg_until_parser::operator() returns NEED_MORE_INPUT at EOF only in lenient mode, then falls through to SUCCESS in final mode. The behavior dates to the PEG combinator introduction in 0a8026e768.