ci: standardize workflow and re-enable tests

[phmatray]

Summary

• Re-enable the commented-out test job and add it back to deploy dependencies
• Add concurrency group (ci-${{ github.ref }}) to cancel redundant workflow runs
• Update checkout to v6, codecov-action to v5
• Standardize NuGet secret name from NUGET_APIKEY to NUGET_API_KEY

Test plan

• Verify CI workflow triggers on push to main
• Verify test job runs and passes
• Verify deploy job waits for test job completion