Add test coverage reporting with Codecov

[osteele]

Description

Add test coverage reporting to CI workflow using Codecov, similar to the liquid project's implementation.

Benefits

• Track test coverage trends over time
• Identify untested code paths
• Prevent coverage regressions in PRs
• Visualize coverage with badges

Implementation

Add a new coverage job to .github/workflows/go.yml that:

1. Runs after tests pass
2. Only runs on pushes (not PRs to avoid duplicate runs)
3. Generates coverage report with go test -coverprofile=coverage.out ./...
4. Uploads to Codecov using codecov/codecov-action@v4
5. Set fail_ci_if_error: false to not block on Codecov issues

Reference

See liquid project's coverage job: https://github.com/osteele/liquid/blob/main/.github/workflows/go.yml#L40-L66