Improve index label construction

The logic is not always consistent, sometimes we get an `IndexLabel`, sometimes a `String`.
The logic is also scattered into multiple files

Start by locating IndexLabel and every place index labels are constructed or converted to String; the issue names no files or tests, so map the scattered logic before deciding scope. Done means index-label construction follows one consistent representation across those call sites, with tests covering the resulting behavior.

Written by the indexing model from the issue text.