Backlog: Introduce edge-case matrix checklist for high-risk refactors

[lipkau]

Problem\nRefactors in transport and response paths can regress edge behavior even when happy-path tests pass.\n\n## Proposal\nCreate a lightweight edge-case matrix checklist for refactors touching request context, session handling, caching, and paging.\n\n## Acceptance criteria\n- Checklist enumerates required edge-path test cases.\n- It is reusable across future refactors.\n- It is referenced by contribution/testing guidance.