chore: remove mocha, chai, and c8

[theorderingmachine]

Summary

• remove mocha/chai/c8 and their type packages from devDependencies
• switch unit tests to Node's built-in test runner and coverage
• add lightweight test globals/assertion shims so the existing suite keeps working

Testing

• nix shell nixpkgs#nodejs -c bash -lc "export NODE_ENV=development && npm run test:coverage -- --test-concurrency=1"

Notes

• I ran the test suite locally in a Nix-provided Node environment.
• I did not run npm run lint locally because the repo's vendored Biome binary does not execute under this NixOS environment; CI can validate that path on GitHub.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @theorderingmachine to sign the Salesforce Inc. Contributor License Agreement.

[changeset-bot]

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