Prevent AI making short unit test helper functions

[CharlieL7]

Motivation

• Make it so AI tools try to keep unit tests self contained rather than having many short helper functions.

Technical Details

Changelog Category

Add a CHANGELOG.md entry for any option other than Not Applicable

• • Added: New functionality.
• • Changed: Changes to existing functionality.
• • Removed: Functionality or support that has been removed. (Compared to a previous release)
• • Optimized: Component performance that has been optimized or improved.
• • Resolved Issues: Known issues from a previous version that have been resolved.
• • Not Applicable: This PR is not to be included in the changelog.

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[Copilot]

Pull request overview

Updates the repository’s agent/contributor guidance to discourage introducing small, low-value helper functions inside unit test files, keeping tests more readable and self-contained.

Changes:

• Adds an explicit “Keep tests self-contained” policy for unit tests, including concrete do/don’t examples.
• Clarifies that the only acceptable local helper in a test file is a single run_pass(...) wrapper, with stricter guidance against layering more helpers on top.

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[TedThemistokleous]

LGTM