add Codecov integration for unit test coverage

[kdacosta0]

Summary

• Add unit-tests.yml workflow with Go setup, make test-ci, and Codecov upload (flags: unit, oidc auth)
• Add test-ci Makefile target with race detector and coverage profiling (-coverprofile=coverage.out -covermode=atomic)
• Add codecov.yml with target: auto, informational: true, and carryforward: true

Test plan

• Verify make test-ci runs locally and produces coverage.out
• Verify CI workflow triggers on push and uploads coverage to Codecov

🤖 Generated with Claude Code

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️