fix: guard against NaN line-rate in Cobertura when no lines tracked

[mcarvin8]

Summary

• finalize() in CoberturaCoverageHandler computed package and global @line-rate via division without guarding for a zero denominator
• A file with no tracked lines (empty lines object) produced NaN in both values, silently breaking every downstream tool that reads line-rate from the Cobertura XML (Codecov, Azure DevOps, Jenkins, GitLab, etc.)
• Both division sites now return 0 when the denominator is zero

Test plan

• Existing empty-file test extended to assert pkg['@line-rate'] and result.coverage['@line-rate'] are 0, not NaN
• All 104 unit tests pass (npm run test:only)
• Branch coverage 100% (192/192)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/handlers/cobertura.ts	100.00% <100.00%> (ø)

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