Add Codecov integration for test coverage tracking

Set up [Codecov](https://codecov.io/) to track and visualize test coverage for the plugin.

Original Request/Idea: https://github.com/WordPress/two-factor/issues/468#issuecomment-3854425118
Other notable implementations ([WP AI Client](https://github.com/WordPress/wp-ai-client/pull/51), [Plugin Check](https://github.com/WordPress/plugin-check))