test(grep): cover the numeric pattern that -l silently swallowed

A non-numeric pattern stops at clap's parse error and falls back to raw
grep, so rtk and grep agree byte-for-byte whether or not `-l` is bound to
--max-len: the three existing cases still pass with the bug reintroduced.
Only a numeric pattern reaches the wrong answer -- rtk printed nothing and
exited 1 where grep listed the file.

Drop the clap-level test and the comment sentence with it. Develop's
test_grep_parse_files_with_matches_l already fails under the same mutation,
and -L never had a short binding on --max-len, so src/main.rs is untouched.

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