add code coverage report into PRs

As discussed in PR #415, we should add code coverage reports into PRs as part of the test workflow.

possible implementation with [Code Coverage Report](https://github.com/marketplace/actions/code-coverage-report-action):

```
- name: Produce the coverage report
uses: insightsengineering/coverage-action@v2
with:
path: coverage.xml
# Publish the rendered output as a PR comment
publish: true
# Create a coverage diff report.
diff: true
# Branch to diff against.
diff-branch: next
# This is where the coverage reports for the
# `diff-branch` are stored.
diff-storage: _xml_coverage_reports
togglable-report: true
```

Review PR #415 and locate the repository's test workflow, then compare its existing coverage output with the proposed insightsengineering/coverage-action@v2 configuration. Done means pull requests receive a rendered coverage comment and coverage diff using coverage.xml, with the configured next branch and storage behavior.

Written by the indexing model from the issue text.