Enable code coverage analysis locally and in CI

[rwood-moz]

Enable code coverage analysis (for the backend tests) both when running the tests locally as well as in CI. See the updated README file for more information.

Thanks @MelissaAutumn as you already had the groundwork for this in the repo! I am just updating the README and enabling the code coverage tool to run in CI.

[rwood-moz]

Got the code coverage report working both locally and in CI now \o/

Here's a link to a GHA validate-backend CI run and you can see the code coverage report:
https://github.com/thunderbird/appointment/actions/runs/12933592396/job/36072579769

Note: I am setting the continue-on-error: true flag for the generate code coverage report step because if the report fails or code coverage is under the threshold (70%) I don't want that to fail the entire workflow and block deployments.

[MelissaAutumn]

Looks good! Some minor things but nothing thats not a 2 second fix.

Do we have a way to view the coveeage level from the pr?

[devmount]

Awesome work, thank you Rob!

[devmount]

Nice! This should fix some issues we had due to different python versions 👍🏻

[rwood-moz]

Looks good! Some minor things but nothing thats not a 2 second fix.

Do we have a way to view the coveeage level from the pr?

There are some third-party github actions that allow just that - publish your code coverage results to a PR comment, i.e.:

https://github.com/marketplace/actions/python-coverage

However I'm not sure if we want to use third-party GHA that aren't certified by GitHub because of security; also it requires a separate github token also. Maybe something to consider in the future though!