Add ambiguity detection tests (#6053)

# Objective

- Add unit tests for ambiguity detection reporting.
- Incremental implementation of #4299.

## Solution

- Refactor ambiguity detection internals to make it testable. As a bonus, this should make it easier to extend in the future.

## Notes

* This code was copy-pasted from #4299 and modified. Credit goes to @alice-i-cecile and @afonsolage, though I'm not sure who wrote what at this point.