Add Codecov coverage for Go SDK

[hubcio]

Rust and Java already upload coverage - Go is missing. Coverage is already generated (reports/go-coverage.out) but never uploaded.

• Add codecov/codecov-action@v5 upload step in .github/workflows/_test.yml for sdk-go with flags: go, file reports/go-coverage.out
• Add post-merge baseline job in .github/workflows/post-merge.yml (same pattern as java-coverage — start server, run tests with coverage, upload)
• Add go flag to codecov.yml (paths: foreign/go/) and remove foreign/go/** from ignore

[atharvalade]

can someone assign this issue to me?