Add Codecov coverage for Node SDK

[hubcio]

Rust and Java already upload coverage - Node is missing. Follow the same pattern.

• Generate lcov output: either add c8 as devDep or use Node 23 native --test-reporter=lcov --test-reporter-destination=../../reports/node-coverage/lcov.info in .github/actions/node-npm/pre-merge/action.yml
• Add codecov/codecov-action@v5 upload step in .github/workflows/_test.yml for sdk-node with flags: node
• Add post-merge baseline job in .github/workflows/post-merge.yml (same pattern as java-coverage — start server, run tests with coverage, upload)
• Add node flag to codecov.yml (paths: foreign/node/) and remove foreign/node/** from ignore