dev-0.43.0-rc.280

refactor(grep): extract error-exit decision into pure unit-tested fn

Address KuSh review on #2465:
- Move the `exit_code >= 2` decision into a pure `is_grep_error_exit`
  function and unit-test it directly (0/1 = normal, >=2 = error),
  instead of faking the rg binary in an integration test.
- Drop the GREP_ERROR_EXIT const; the doc comment on the function
  conveys the grep/rg exit convention.
- Remove tests/grep_error_test.rs (faked binary) in favour of the
  unit test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>