Improve test coverage to 92% on CodeCov

[DecisionNerd]

Problem

Our test coverage on CodeCov is currently at ~88%, which is dangerously close to the 85% minimum threshold. We need to improve coverage to at least 92% to provide a safer buffer.

Current Status

• Current coverage: ~87.95%
• Minimum threshold: 85% (enforced by CI)
• Target coverage: 92%
• Gap to close: ~4%

Why This Matters

1. Close to threshold means small changes can cause CI failures
2. Low coverage areas are more likely to have bugs
3. Better coverage improves confidence in refactoring
4. Establishes best practices for future development

Approach

1. Identify uncovered code sections using coverage reports
2. Prioritize critical paths and complex logic
3. Add targeted unit tests for uncovered branches
4. Focus on edge cases and error handling paths

Success Criteria

• Total coverage reaches 92% or higher
• All critical paths have coverage
• Coverage reports uploaded to CodeCov successfully
• CI passes with new threshold

Related

• PR feat: implement temporal truncate() and arithmetic functions #135 (temporal arithmetic) currently at 87.95% coverage
• Local coverage threshold: 85% (pyproject.toml)
• CodeCov patch coverage: 75% minimum