Render subdiagnostics in the Ruff/ty playgrounds (#25490)
## Summary
@decorator-factory
[said](https://discord.com/channels/267624335836053506/891788761371906108/1510258224719204352)
on the Python community Discord that
> Maybe better error messages are a good reason to recommend pyright
over other things.
It turns out they'd been evaluating ty's diagnostics purely on the
diagnostics they could see in the ty playground. That seems a bit of a
shame, since we've improved our diagnostics a lot recently thanks to
@sharkdp's excellent work -- but none of those improvements are visible
in the playground right now, as the playground doesn't render any
subdiagnostics currently!
This PR adds subdiagnostics to the Ruff and ty playgrounds: both in the
"diagnostics" panel at the bottom of the playground, and in the tooltip
rendering when you hover over a line with a diagnostic. In both cases,
subdiagnostics with annotations are clickable, allowing you to jump
straight to the line of code being annotated (even if it's in another
file, even if that file is a vendored typeshed stub).
Obviously I'm very much not a frontend developer; this patch was very
much Codex-driven. I manually tested it, however, and did several
`/review` and `$simplify` rounds with Codex before submitting this.
## Test Plan
https://github.com/user-attachments/assets/3418d337-0f34-4e9f-8203-3da2f2008b86
