Add rule code to GitLab description (#19896)
## Summary
Fixes #19881. While I was here, I also made a couple of related tweaks
to the output format. First, we don't need to strip the `SyntaxError: `
prefix anymore since that's not added directly to the diagnostic message
after #19644. Second, we can use `secondary_code_or_id` to fall back on
the lint ID for syntax errors, which changes the `check_name` from
`syntax-error` to `invalid-syntax`. And then the main change requested
in the issue, prepending the `check_name` to the description.
## Test Plan
Existing tests and a new screenshot from GitLab:
<img width="362" height="113" alt="image"
src="https://github.com/user-attachments/assets/97654ad4-a639-4489-8c90-8661c7355097"
/>
