[ty] Add workflow to comment conformance tests diff (#19555)
## Summary
Follow-up to https://github.com/astral-sh/ruff/pull/19556, this PR adds
the workflow that computes the diagnostic diff which the workflow
introduced in the linked PR will add as a comment.
This workflow is similar to the [ty ecosystem-analyzer
workflow](https://github.com/astral-sh/ruff/blob/d781a6ab3f44afe5dd1afe1d06fb58fd3739fab5/.github/workflows/ty-ecosystem-analyzer.yaml).
Closes: astral-sh/ty#212
## Test Plan
1. Initially there's no diff to show
2. This
[commit](https://github.com/astral-sh/ruff/pull/19555/commits/d0db9937df29cf2d192025ca8f9e480e4acc26f3)
comments out a rule which updates the comment with the diff
3. Later, that commit is reverted and the diff goes away
Use the comment history to look at the diff output where the order of
the history corresponds to the steps mentioned above in reverse order
i.e., the edit in the middle will contain the diff output:
<img width="1082" height="313" alt="Screenshot 2025-07-25 at 21 09 26"
src="https://github.com/user-attachments/assets/6aceb60c-1987-4b9a-9063-e3999844f035"
/>
