Add comprehensive test coverage

Ensure all code changes include appropriate unit tests and address any identified testing gaps immediately. When reviewing code, verify that:

1. New functionality has corresponding tests: Every new feature or significant code change should include unit tests that exercise the new behavior.

2. Existing untested code gets tests when modified: If you’re working on code that lacks test coverage, add tests as part of your changes rather than deferring them.

3. Test TODOs are addressed, not accumulated: Don’t leave comments like “TODO: This has a bug!!!” in test code. Either fix the issue immediately or create a proper ticket to track it.

4. Mark missing test areas explicitly: When you identify code that needs tests but can’t add them immediately, add clear TODO comments like (* TODO: test this *) to acknowledge the gap and help future developers.

Example from the discussions:

(* Instead of leaving this unaddressed: *)
(* TODO: This has a bug!!!
   Suppose we exclude `test.py` and are given `tests2/test.py`.
   This code will not exclude properly... *)

(* Do this: *)
(* Add unit test for path exclusion edge cases - see issue #1234 *)
let test_path_exclusion_with_nested_dirs () = 
  (* Test the specific bug case mentioned *)
  assert (excludes_file "test.py" "tests2/test.py" = true)

The goal is to build confidence in code changes through comprehensive testing rather than relying on manual verification like “run -cfg_il and looks like it’s ok”.