fix(search): make format-flag detection engine-aware for -L/-z

is_format_flag_token (and the has_format_flag/extract_pattern_path call
sites) treated -L and -z identically for grep and rg, but confirmed
against real grep --help/rg --help that they mean unrelated things per
engine: grep's -L is --files-without-match (a real shape/format flag)
and -z is --null-data (input line terminator), while rg's -L is
--follow (symlinks) and -z is --search-zip (search compressed files) --
neither an rg format flag. `rtk rg -L pattern src` was misdetected as
already-minimal output and wrongly routed to raw passthrough, skipping
RTK's own match compression/grouping. Threaded Engine through
extract_pattern_path/is_format_flag_token so the check is per-engine.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>