Backlog: Add PS5-safe test helper checklist for Add-Type and analyzer compatibility

[lipkau]

Problem\nTests can pass locally on PowerShell 7 but fail on PSv5 due to stricter Add-Type/C# constraints and analyzer expectations.\n\n## Proposal\nAdd a checklist for writing/updating test helpers that are PS5-compatible, including Add-Type patterns and common analyzer pitfalls.\n\n## Acceptance criteria\n- Checklist covers PS5 C# compatibility gotchas with examples.\n- Checklist includes analyzer-sensitive test patterns used in this repo.\n- Guidance is linked from test authoring docs.