Add Codecov coverage reporting and badge

[larkiny]

Summary

• Upload coverage JSON to Codecov after test-ts job using codecov/codecov-action@v5
• Add JSON coverage reporter alongside text output
• Add Codecov badge to README
• Add coverage/ to .gitignore

Test plan

• CI uploads coverage to Codecov successfully
• Codecov badge renders on README after first successful upload
• Coverage table still prints in CI logs

[codecov]

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.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

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