🗒️ Description

The coverage CI doesn't handle cases where we modify existing ported tests correctly. This PR resolves that issue.

Problem

When modifying an existing test file that contains @pytest.mark.ported_from markers from past porting, the coverage CI would:

• Detect existing ported markers in the modified file
• Set any_ported=true
• Skip generating BASE_TESTS for coverage comparison
• Fail when trying to read non-existent BASE_TESTS index files

Solution

This fix operates on the principle that once a test is ported, the Python version becomes the source of truth. We distinguish between:

• New porting operations: New @pytest.mark.ported_from markers in the git diff, means we only compare against original JSON from ethereum/tests.
• Modifications to existing ported tests: No new @pytest.mark.ported_from markers in git diff, means we compare against the previous Python version.

Future Considerations

The coverage script is not capable of handling cases where new ported tests are added to an existing file that modifies existing tests. The recommendation for porting any new test from ethereum/tests is to a) add it to a new file but b) at a minimum do not modify any existing tests if added to an existing file.

🔗 Related Issues or PRs

#2002

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx --with=tox-uv tox -e lint,typecheck,spellcheck,markdownlint

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).