Add toInclude() function

Editor assessment

`feat: add toInclude() function (#2234)` names the added function exactly, which is the essential fact for a test-helper addition; readers of the test suite know immediately what appeared. Feat is defensible for new API surface, and no scope is needed. The body carries only co-author attribution, properly formatted. No body explains the helper's semantics — membership assertion — a modest gap, but the function name itself communicates. Imperative verb, no trailing period, compact. Clear, correct, and self-contained: a publishable example.