[Rule] Behavior of existing API has changed

There is a functional change in what the API was doing. This will need to be determined on a case by case basis.

The issue names no file, test, or entry point. First identify the rule handling functional changes in the OpenAPI comparison CLI, then determine the behavior case by case; the work is done only when the intended behavior is agreed and represented consistently.

索引モデルが issue の本文から書いたものです。