Unify `Message` variants (#18051)
## Summary
This PR unifies the ruff `Message` enum variants for syntax errors and
rule violations into a single `Message` struct consisting of a shared
`db::Diagnostic` and some additional, optional fields used for some rule
violations.
This version of `Message` is nearly a drop-in replacement for
`ruff_diagnostics::Diagnostic`, which is the next step I have in mind
for the refactor.
I think this is also a useful checkpoint because we could possibly add
some of these optional fields to the new `Diagnostic` type. I think
we've previously discussed wanting support for `Fix`es, but the other
fields seem less relevant, so we may just need to preserve the `Message`
wrapper for a bit longer.
## Test plan
Existing tests
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Co-authored-by: Micha Reiser <micha@reiser.io>
