[red-knot] Fix stale syntax errors in playground (#17280)
## Summary
React requires that `key`s are unique but the constructed key for
diagnostics wasn't guaranteed when two diagnostics had the same name and
location.
This PR fixes this by using a disambiguator map to disambiguate the key.
Fixes #17276
## Test Plan
https://github.com/user-attachments/assets/f3f9d10a-ecc4-4ffe-8676-3633a12e07ce
