[ty] Fix Salsa panic propagation (#24141)
## Summary
This PR fixes probably the most likely case why users saw
https://github.com/astral-sh/ty/issues/1565 in their IDE.
I added handling to convert panics to diagnostics in
https://github.com/astral-sh/ruff/pull/17631 to `check_file_impl`.
However, this was before `check_file_impl` became a Salsa query. We have
to be a bit more careful, now that `check_file_impl` is a Salsa query.
1. We have to add an untracked read whenever we suppress a panic because
Salsa only carries over dependencies of queries that run to completion
and not of queries that panic (the assumption is that all queries
unwind). Adding an `untracked_read` ensures that the `check_file_impl`
reruns after every change. This is more often than necessary, but it is
the best we can do here without knowing the exact dependencies that were
collected up to when `check_file_impl`'s dependency panicked.
2. Suppressing all Salsa panics in `check_files_impl` is unlikely to be
what we want because it means the function still runs to completion even
when the query was cancelled. Instead, we want to propagate a
cancellation so that its `db` handle gets released as quickly as
possible to unblock any pending mutation. However, we do need some
special handling for Salsa's propagating panic to avoid regressing
https://github.com/astral-sh/ruff/pull/17631. For a query `A` running on
thread `a` that depends on query `B` running on thread `b`. If `B`
panics, Salsa throws the original panic on thread `b` but throws a
`Cancelled::PropagatingPanic` panic on thread `a`. Thread `b`'s panic is
the more useful one because it contains the actual panic information. We
already convert `b`'s panic to a `Diagnostic`, but we silently ignore
any `Cancelled::PropagatingPanic`. This PR also creates a propagating
panic to a diagnostic. While these panics don't contain any useful
information, they at least indicate to a user that `A` was only
partially checked. They should have a second diagnostic for `B` that
contains the full panic information (unless `B` is a query that didn't
run as part of `project.check`, e.g. `hover`).
## Test plan
I used @AlexWaygood's panda-stubs reproducer and I was no longer able to
reproduce the Salsa panic.
I plan on doing some CLI `--watch` testing tomorrow morning but this
should block code review.
