Implement coverage check in CI

[mum4k]

We should implement a CI check that will generate HTML coverage report to help with PR reviews and verify both total and incremental coverage against set thresholds. This will ensure that:

• newly added code has sufficient coverage
• total coverage in the salvo codebase doesn't regress below our goal

[gyohuangxin]

I think Codecov will be the tool to implement it, and here is the example we can refer to: https://about.codecov.io/blog/python-code-coverage-using-github-actions-and-codecov/

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had activity in the last 90 days. It will be closed in the next 14 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[github-actions[bot]]

This issue has been automatically closed because it has not had activity in the last 104 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted" or "no stalebot". Thank you for your contributions.