Upload junit reports codecov

[aleguy02]

Description

This PR automates uploading JUnit reports to codecov in the test pipeline. An example pipeline run with the changes can be found here (https://github.com/aleguy02/fork-pytest/actions/runs/18184970357). An example of what the codecov UI looks like with these changes can be found here (https://app.codecov.io/gh/aleguy02/fork-pytest/tests).

Closes #12689

Checklist

• [ ] Include documentation when adding new features.
• [ ] Include new tests or update existing tests when applicable.
• [X] Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

Additional Notes

I didn't create a changelog since the documented behavior of pytest was not modified.