test(diff): perform boundary analysis and document negative test gaps

[theRebelliousNerd]

This PR introduces a comprehensive boundary value analysis and negative testing strategy for the internal/diff subsystem. It creates a detailed QA journal documenting edge cases (such as the shallow copy pointer trap in the cache, and missing timeouts in massive Myers diffs leading to CPU exhaustion), and appends concrete // TODO: TEST_GAP: tasks to the test suite to guide future remediation.

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PR created automatically by Jules for task 2627604226015219748 started by @theRebelliousNerd

Summary by CodeRabbit

• Documentation

  • Added quality assurance documentation detailing boundary-value analysis and edge-case scenarios for the diff engine, including recommendations for improved reliability.

• Tests

  • Catalogued missing test coverage for the diff engine, including edge cases for empty inputs, binary content, cache behavior, concurrency, and resource exhaustion scenarios.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9fec7162-4c0d-4b8c-82a4-c20c8c5649e4

📥 Commits

Reviewing files that changed from the base of the PR and between 664e51f and 5cb4d93.

📒 Files selected for processing (2)

• .quality_assurance/2026-05-22_00-21-EST_diff_boundary_analysis.md
• internal/diff/diff_test.go

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📝 Walkthrough

Walkthrough

This PR adds a comprehensive QA journal documenting boundary-value analysis and negative test gaps for the internal diff engine, alongside test TODO comments outlining missing test coverage. The journal catalogs edge cases including empty inputs, binary content, cache collisions, hunk boundaries, large single-line inputs, timeouts, resource exhaustion, and concurrency races, with proposed remediation steps.

Changes

Diff Engine Test Coverage Planning

Layer / File(s)

Summary

Boundary-value analysis journal and test coverage TODOs .quality_assurance/2026-05-22_00-21-EST_diff_boundary_analysis.md, internal/diff/diff_test.go

A new QA journal systematically documents edge cases across empty inputs, binary/non-UTF-8 content, cache hashing collisions, context/hunk boundaries, large single-line "minified" inputs, diff timeouts, resource exhaustion, concurrency/cache corruption, and newline handling. The test file is updated with TODO comments enumerating concrete missing test cases (empty strings, newline-at-EOF, hash collision cache behavior, binary null bytes, invalid UTF-8, shallow-copy pointer safety, ClearCache races, and context merging boundary correctness) to be added to diff_test.go.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

🐰 A journal of edge cases hops into the fold,
Where empty diffs and binary treasures unfold,
With TODOs marking gaps to fill with care,
The diff engine's boundaries laid bare!
Test vectors waiting, remedies planned—
Quality awaits at the testing strand! 🧪✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: performing boundary analysis and documenting negative test gaps in the diff subsystem.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch jules-diff-boundary-analysis-2627604226015219748

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