test: improve test coverage to 90%+

[GeiserX]

Summary

• Expand test suite from 17 tests (22% coverage) to 269 tests (94% total coverage)
• Add 3 new test files: test_link_traverser.py, test_report_generator.py, test_visual_comparison.py
• Rewrite 4 existing test files with comprehensive coverage: test_cli.py, test_diff_engine.py, test_fetcher.py, test_wayback_cleaner.py
• Fix 2 bugs found during testing:
  • diff_engine.py: compare_structures TypeError on Python 3.14 (SequenceMatcher rejects unhashable dict elements)
  • wayback_cleaner.py: remove_wayback_header fallback path skipping content past the <meta> tag boundary

Per-module coverage

Module	Coverage
__init__.py	100%
cli.py	96%
diff_engine.py	97%
fetcher.py	95%
link_traverser.py	89%
report_generator.py	100%
visual_comparison.py	90%
wayback_cleaner.py	99%
TOTAL	94%

Test plan

• All 269 tests pass locally with pytest --cov=wayback_diff
• Total coverage 94% (target was 90%+)
• No real network requests — all HTTP/browser interactions are mocked
• CI pipeline confirms coverage meets codecov.yml 90% threshold

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 9f103cc7-06e7-4dde-aaf2-78a777e7036e

📥 Commits

Reviewing files that changed from the base of the PR and between 5e09d17 and 6fec90a.

📒 Files selected for processing (12)

• requirements-dev.txt
• requirements.txt
• tests/test_cli.py
• tests/test_diff_engine.py
• tests/test_fetcher.py
• tests/test_link_traverser.py
• tests/test_report_generator.py
• tests/test_visual_comparison.py
• tests/test_wayback_cleaner.py
• wayback_diff/diff_engine.py
• wayback_diff/visual_comparison.py
• wayback_diff/wayback_cleaner.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/improve-coverage

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[codecov]

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