This PR introduces the bare minimum for code coverage reporting using cargo-llvm-cov and integrates with Codecov. It adds a new coverage job to the CI workflow that generates and uploads coverage reporting in parallel with existing tests. The setup mirrors that of Router, except it uses nextest instead of the built-in test runner and CircleCI instead of GitHub Actions.

For more details, check out the last successful workflow run here: https://github.com/apollographql/apollo-mcp-server/actions/runs/17557278384/job/49864559944

You can also view the initial coverage generated from this feature branch on Codecov here: https://app.codecov.io/gh/apollographql/apollo-mcp-server

If you want to check coverage locally, run this command in the terminal. It will open the coverage report in your browse

$ cargo llvm-cov --all-features --workspace --open

If you've worked on Router, you should already have the llvm-cov subcommand installed. If not, you can install it as follows:

$ cargo install cargo-llvm-cov

Once this PR is merged into develop and we start collecting coverage there, I'll follow up with additional settings such as thresholds, docs, badges, and more.