ruff

981bd70d - Convert `Message::SyntaxError` to use `Diagnostic` internally (#17784)

Convert `Message::SyntaxError` to use `Diagnostic` internally (#17784) ## Summary This PR is a first step toward integration of the new `Diagnostic` type into ruff. There are two main changes: - A new `UnifiedFile` enum wrapping `File` for red-knot and a `SourceFile` for ruff - ruff's `Message::SyntaxError` variant is now a `Diagnostic` instead of a `SyntaxErrorMessage` The second of these changes was mostly just a proof of concept for the first, and it went pretty smoothly. Converting `DiagnosticMessage`s will be most of the work in replacing `Message` entirely. ## Test Plan Existing tests, which show no changes. --------- Co-authored-by: Carl Meyer <carl@astral.sh> Co-authored-by: Micha Reiser <micha@reiser.io>