Methods that return only lists or arrays should use 'IReadOnlyList<T>' instead of 'IEnumerable<T>' as return type

[RalfKoban]

If a method only returns a list or array, avoid using IEnumerable<T> as the return type. Instead, prefer returning IReadOnlyList<T>.

This makes it clear to consumers that:

1. The result is a complete, finite collection, not a potentially lazy or infinite sequence.
2. They can use the result directly without needing to call .ToList() to materialize it.

Using IReadOnlyList<T> improves clarity and usability, especially when the method is guaranteed to return a fully realized list.

Note: Do not report that issue in case the method is an interface implementation or an override.