Sharing PR Modeling

After fixing the acute issue in LintDiff and discovering and fixing a couple more failure scenarios, let's have a closer look at how LintDiff models PRs and whether we can use that capability in other places like Breaking Changes.

* [x] Parameterize tests or come up with a Domain-Specific Language?~ DONE
* [ ] Is there some overlap with Breaking Changes?
* [ ] There's an opportunity for a "PRModel" which can compare before/after in 2 different SpecModel objects and give useful results
* [ ] Maybe factor the tests so that the scenarios can be easily extracted and used in other tests

_Originally posted by @danieljurek in https://github.com/Azure/azure-rest-api-specs/issues/38698#issuecomment-3560147134_